gdb/testsuite: Make clean mostlyclean should not delete *.py.

gdb/testsuite: Make clean mostlyclean should not delete *.py.

A sanity-check in my release scripts caught something: After having
created the tarballs, I verify that no checked-in file disappeared
in the process, and lo and behod, it found that the following file
got wiped:

    - gdb/testsuite/dg-extract-results.py:

And it's not part of the tarball either.

I don't understand while we delete all *.py files in gdb/testsuite,
since I don't see a rule that expected to create one. A run of the
testsuite also doesn't seem to be creating .py files there.
I traced this to the following commit, which unfortunately provided
no explanation. Perhaps we used to run some tests in the gdb/testsuite
directory and caused files to be left behind there. Perhaps we still
do today?

In the meantime, Executive Decision: In order to allow me to create
tarballs without losing files, I removed it. It's easy to put something
back if we find out why it might still be needed.

gdb/testsuite/ChangeLog:

        * Makefile.in (clean mostlyclean): Do not delete *.py.

Tested on x86_64-linux by running the src-release.sh script again,
and this time, dg-extract-results.py no longer gets wiped.