[build] Custom GDB no longer needed in Ubuntu Bionic

Makefile

@@ -323,14 +323,12 @@ test: $(LINUX_BUILD)
 benchmark: $(LINUX_BUILD)
 	$(NINJA) $(NINJA_ARGS) -j$(JOBS) -C $(LINUX_OUTPUT_PATH) mbgl-benchmark
 
-ifneq (,$(shell command -v gdb 2> /dev/null))
-  GDB ?= $(shell scripts/mason.sh PREFIX gdb VERSION 2017-04-08-aebcde5)/bin/gdb \
-        	-batch -return-child-result \
-        	-ex 'set print thread-events off' \
-        	-ex 'set disable-randomization off' \
-        	-ex 'run' \
-        	-ex 'thread apply all bt' --args
-endif
+GDB ?= gdb \
+	-batch -return-child-result \
+	-ex 'set print thread-events off' \
+	-ex 'set disable-randomization off' \
+	-ex 'run' \
+	-ex 'thread apply all bt' --args
 
 .PHONY: run-test
 run-test: run-test-*