Remove support for the useless 'custom' option in SYSCALLS.TXT.

This cruft dates from a time when bionic would only output syscall
contants for the syscalls mentioned in SYSCALLS.TXT. I fixed that
a long time ago, but never followed through with the removal of what
was then confusingly called "stub" and was recently renamed "custom".

Change-Id: I8f3872a200b2dc8325e357cc5ee505ea4212ef95

libc/tools/bionic_utils.py

@@ -122,9 +122,7 @@ class SysCallsTxtParser:
 
         # Parse the architecture list.
         arch_list = line[pos_rparen+1:].strip()
-        if arch_list == "custom":
-            pass
-        elif arch_list == "all":
+        if arch_list == "all":
             for arch in all_arches:
                 t[arch] = True
         else: