Remove dead code from _get_platforms

Since manylinux* is only possible if provided by the user directly.

Author: chrahunt

src/pip/_internal/pep425tags.py

@@ -320,8 +320,6 @@ def _get_platforms():
     arch_prefix, arch_sep, arch_suffix = arch.partition('_')
     if arch.startswith('macosx'):
         arches = _mac_platforms(arch)
-    elif arch_prefix in ['manylinux2014', 'manylinux2010']:
-        arches = _custom_manylinux_platforms(arch)
     else:
         arches = []
         if is_manylinux2014_compatible():