tests: sensor SBS EMUL_CONFIG to testcase.yaml

There exists several <board>.conf files in the boards directory of the
sbs_gauge tests. All these confs do is enable CONFIG_EMUL. We can remove
these board confs in favor of a single extra_configs entry in the sensor
sbs gauge testcase.yaml file.

Remove the sensor test_bs_gauge test's board conf files in favor of
enabling CONFIG_EMUL in the testcase.yaml file.

Signed-off-by: Aaron Massey <aaronmassey@google.com>