[gdb/testsuite] Drop global decls at global level

While reviewing a patch I came across:
...
global srcdir
...

This is only required in a proc, so this is superfluous.

Delete similar cases using:
...
$ find gdb/testsuite/gdb.* -name *.exp | xargs sed -i '/^global /d'
...

Tested on x86_64-linux.

Approved-By: Luis Machado <luis.machado.foss@gmail.com>