gdb/Makefile.in: remove SOURCES variable

The SOURCES variable was added to gdb/Makefile.in as part of commit:

  commit fb40c20903110ed8af9701ce7c2635abd3770d52
  Date:   Wed Feb 23 00:25:43 2000 +0000

      Add mi/ and testsuite/gdb.mi/ subdirectories.

But as far as I can tell was not used at the time it was added, and is
not used today.

Lets remove it.