Simplify _get_custom_platforms

Since we only call this function when platform is not None, we can drop
one of the branches. This is where we were originally populating
manylinux arches, so those can be removed next.

Author: chrahunt

src/pip/_internal/pep425tags.py

@@ -296,22 +296,13 @@ def _custom_manylinux_platforms(arch):
     return arches
 
 
-def _get_custom_platforms(arch, platform):
-    # type: (str, Optional[str]) -> List[str]
+def _get_custom_platforms(arch):
+    # type: (str) -> List[str]
     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)
-    elif platform 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
@@ -367,7 +358,7 @@ def get_supported(
 
     platforms = None  # type: Optional[List[str]]
     if platform is not None:
-        platforms = _get_custom_platforms(platform, platform)
+        platforms = _get_custom_platforms(platform)
 
     is_cpython = (impl or interpreter_name()) == "cp"
     if is_cpython: