hgrc: search XDG_CONFIG_HOME on mac

Searching for hgrc was special cased not to look through ~/.config/hg on Mac,
but that’s unnecessary: Macs support it as do other unix based systems.  There
are plenty tools that use it there, e.g. git, and people expect it to work, e.g.

"https://stackoverflow.com/questions/72499837/mercurial-on-macos-doesnt-read-config-hg-hgrc".

Initial code introduced in 354020079723.

mercurial/scmposix.py

@@ -60,8 +60,6 b' def systemrcpath() -> List[bytes]:'
 def userrcpath() -> List[bytes]:
     if pycompat.sysplatform == b'plan9':
         return [encoding.environ[b'home'] + b'/lib/hgrc']
-    elif pycompat.isdarwin:
-        return [os.path.expanduser(b'~/.hgrc')]
     else:
         confighome = encoding.environ.get(b'XDG_CONFIG_HOME')
         if confighome is None or not os.path.isabs(confighome):

tests/test-xdg.t

@@ -1,4 +1,4 b''
-#if no-windows no-osx
+#if no-windows
 
   $ mkdir -p xdgconf/hg
   $ echo '[ui]' > xdgconf/hg/hgrc