packfile: delete index maps from memory when no longer needed

[strib]

This helps keep memory usage stable while calculating deltas.

[codecov]

Codecov Report

Merging #632 into master will decrease coverage by 0.57%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #632      +/-   ##
=========================================
- Coverage   78.28%   77.7%   -0.58%     
=========================================
  Files         130     130              
  Lines       10221   10223       +2     
=========================================
- Hits         8001    7944      -57     
- Misses       1358    1430      +72     
+ Partials      862     849      -13

Impacted Files	Coverage Δ
plumbing/format/packfile/delta_selector.go	80.36% <100%> (+0.24%)	⬆️
plumbing/transport/ssh/common.go	20.54% <0%> (-45.21%)	⬇️
plumbing/transport/ssh/auth_method.go	31.57% <0%> (-22.81%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 50732e3...1e9ad3e. Read the comment docs.