[gdb/testsuite] Fix unset of DEBUGINFOD_URLS in default_gdb_init

[gdb/testsuite] Fix unset of DEBUGINFOD_URLS in default_gdb_init

In commit cfcbd506fb0 "[gdb/testsuite] Ignore DEBUGINFOD_URLS" I added
unsetting of env(DEBUGINFOD_URLS), but it doesn't work because I forgot to
add :: in front.

Fix this, and rewrite using "unset -nocomplain" instead of unsetenv, which
allows us to drop the "info exists" test.

2021-04-01  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (default_gdb_init): Use ::env.  Use unset
	-nocomplain ::env(V) instead of unsetenv V.