Remove string_to_utc_time conversion function

This method was used in features that were removed (#268 and #271) and
the reason for converting ts values selected from the timeline query
is not explicitly documented in the commit history and appears to
be redundant (they are already timestamps)

After conducting manual tests on the `usec`s, it was determined that
the issue reported in issue #32 is no longer valid.

lib/chrono_model/conversions.rb

@@ -6,19 +6,6 @@ module Conversions
 
     ISO_DATETIME = /\A(\d{4})-(\d\d)-(\d\d) (\d\d):(\d\d):(\d\d)(?:\.(\d+))?\z/
 
-    # rubocop:disable Style/PerlBackrefs
-    def string_to_utc_time(string)
-      return string if string.is_a?(Time)
-
-      return unless string =~ ISO_DATETIME
-
-      # .1 is .100000, not .000001
-      usec = $7.ljust(6, '0') unless $7.nil?
-
-      Time.utc $1.to_i, $2.to_i, $3.to_i, $4.to_i, $5.to_i, $6.to_i, usec.to_i
-    end
-    # rubocop:enable Style/PerlBackrefs
-
     def time_to_utc_string(time)
       time.to_fs(:db) << '.' << format('%06d', time.usec)
     end

lib/chrono_model/time_machine/timeline.rb

@@ -81,9 +81,7 @@ def timeline(record = nil, options = {})
         sql << " LIMIT #{options[:limit].to_i}" if options.key?(:limit)
 
         connection.on_schema(Adapter::HISTORY_SCHEMA) do
-          connection.select_values(sql, "#{name} periods").map! do |ts|
-            Conversions.string_to_utc_time ts
-          end
+          connection.select_values(sql, "#{name} periods")
         end
       end
 

spec/chrono_model/conversions_spec.rb

@@ -3,43 +3,6 @@
 require 'spec_helper'
 
 RSpec.describe ChronoModel::Conversions do
-  describe '.string_to_utc_time' do
-    subject(:string_to_utc_time) { described_class.string_to_utc_time(string) }
-
-    context 'with a valid UTC time string' do
-      let(:string) { '2017-02-06 09:46:31.129626' }
-
-      it { is_expected.to be_a(Time) }
-      it { expect(string_to_utc_time.year).to  eq 2017 }
-      it { expect(string_to_utc_time.month).to eq 2 }
-      it { expect(string_to_utc_time.day).to   eq 6 }
-      it { expect(string_to_utc_time.hour).to  eq 9 }
-      it { expect(string_to_utc_time.min).to   eq 46 }
-      it { expect(string_to_utc_time.sec).to   eq 31 }
-      it { expect(string_to_utc_time.usec).to  eq 129_626 } # Ref Issue #32
-    end
-
-    context 'with a valid UTC string without least significant zeros' do
-      let(:string) { '2017-02-06 09:46:31.129' }
-
-      it { is_expected.to be_a(Time) }
-      it { expect(string_to_utc_time.usec).to eq 129_000 } # Ref Issue #32
-    end
-
-    context 'with a valid UTC string without usec' do
-      let(:string) { '2017-02-06 09:46:31' }
-
-      it { is_expected.to be_a(Time) }
-      it { expect(string_to_utc_time.usec).to eq 0 }
-    end
-
-    context 'with an invalid UTC time string' do
-      let(:string) { 'foobar' }
-
-      it { is_expected.to be_nil }
-    end
-  end
-
   describe '.time_to_utc_string' do
     subject { described_class.time_to_utc_string(time) }