[PATCH][gdb/testsuite] Avoid breakpoint in GLIBC in gdb.threads/execl.exp

[Tom Tromey]

>>>>> "Tom" == Tom de Vries <tdevries@suse.de> writes:

Tom> 2020-03-12  Tom de Vries  <tdevries@suse.de>

Tom> 	* gdb.threads/execl.exp: Delete breakpoint after hitting it.

This seems fine to me.  Thank you.
Another way might be to use tbreak, but this seems just as good.

Tom