[tools/dev/gm.py] Remove unused gdb_index.

gdb_index is not in declare_args() and has no effect.

NOTRY=true

Change-Id: I88a9558937aa8fea30ab246899bea4a123947f82
Reviewed-on: https://chromium-review.googlesource.com/475772
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44620}

tools/dev/gm.py

@@ -106,7 +106,6 @@ v8_enable_verify_heap = true
 """.replace("{GOMA}", USE_GOMA)
 
 DEBUG_ARGS_TEMPLATE = """\
-gdb_index = true
 is_component_build = true
 is_debug = true
 symbol_level = 2
@@ -118,7 +117,6 @@ v8_optimized_debug = false
 """.replace("{GOMA}", USE_GOMA)
 
 OPTDEBUG_ARGS_TEMPLATE = """\
-gdb_index = false
 is_component_build = true
 is_debug = true
 symbol_level = 1