Enable optimisation with Asan with make in all.sh

All builds using ASAN_CFLAGS were with Asan but no optimisation, making
them particularly slow. Indeed, we were overwriting CFLAGS which
defaults to -O2 and not using any -O in the replacement. (CMake already has
optimisations on with ASan.)

While at it, also remove -Wall -Wextra which are redundant as they are
already part of WARNING_CFLAGS which we are not overwriting.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index b43f999..f72fe52 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -185,7 +185,7 @@
     export CTEST_OUTPUT_ON_FAILURE=1
 
     # CFLAGS and LDFLAGS for Asan builds that don't use CMake
-    ASAN_CFLAGS='-Werror -Wall -Wextra -fsanitize=address,undefined -fno-sanitize-recover=all'
+    ASAN_CFLAGS='-O2 -Werror -fsanitize=address,undefined -fno-sanitize-recover=all'
 
     # Gather the list of available components. These are the functions
     # defined in this script whose name starts with "component_".