tests/bluetooth/tester: remove spurious CONFIG_GPIO=y

This tries to build gpio without any driver which fails at cmake time
like this:

CMake Error at ../../../cmake/extensions.cmake:378 (add_library):
  No SOURCES given to target: drivers__gpio
  Call Stack (most recent call first):
  ../../../cmake/extensions.cmake:355 (zephyr_library_named)
  ../../../drivers/gpio/CMakeLists.txt:3 (zephyr_library)

Remove TEST_USERSPACE workaround added in commit 3b53e692 which
added gpio_handlers.c to the SOURCES.

Fixes #15232.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>