[gdb/testsuite] Fix duplicate test name in gdb.mi/mi-sym-info.exp

[gdb/testsuite] Fix duplicate test name in gdb.mi/mi-sym-info.exp

Since commit 2d61316c32a "[gdb/testsuite] Fix buffer full errors in
gdb.mi/mi-sym-info.exp", we have a duplicate test name:
...
Running src/gdb/testsuite/gdb.mi/mi-sym-info.exp ...
DUPLICATE: gdb.mi/mi-sym-info.exp: List all variables
...

This is caused by a copy-paste accident: the idea was to copy the test, edit
it, and then remove the old test, but I forgot to remove the old one.
Remove it now.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-04-30  Tom de Vries  <tdevries@suse.de>

	* gdb.mi/mi-sym-info.exp: Remove duplicate test.