Remove dead code from _get_custom_platforms

Since `arch` must be non-`None` for this function to be invoked.

Author: chrahunt

src/pip/_internal/pep425tags.py

@@ -309,15 +309,6 @@ def _get_custom_platforms(arch):
         arches = _mac_platforms(arch)
     elif arch_prefix in ['manylinux2014', 'manylinux2010']:
         arches = _custom_manylinux_platforms(arch)
-    elif arch is None:
-        arches = []
-        if is_manylinux2014_compatible():
-            arches.append('manylinux2014' + arch_sep + arch_suffix)
-        if is_manylinux2010_compatible():
-            arches.append('manylinux2010' + arch_sep + arch_suffix)
-        if is_manylinux1_compatible():
-            arches.append('manylinux1' + arch_sep + arch_suffix)
-        arches.append(arch)
     else:
         arches = [arch]
     return arches