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