[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.

---
 gdb/testsuite/lib/gdb.exp | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 6b0bc8c0af5..ee7358e7673 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -5082,9 +5082,7 @@ proc default_gdb_init { test_file_name } {
 
     # If DEBUGINFOD_URLS is set, gdb will try to download sources and
     # debug info for f.i. system libraries.  Prevent this.
-    if { [info exists env(DEBUGINFOD_URLS)] } {
-	unsetenv DEBUGINFOD_URLS
-    }
+    unset -nocomplain ::env(DEBUGINFOD_URLS)
 
     # Ensure that GDBHISTFILE and GDBHISTSIZE are removed from the
     # environment, we don't want these modifications to the history