[Impeller] Remove check_licenses.py (flutter#52970)

I could not find any users of this. As I understand, this was from a time [when Impeller wasn't a part of the
engine](flutter#97686). Now the engine licenses checkers are sufficient.

Author: chinmaygarde

ci/licenses_golden/excluded_files

@@ -202,7 +202,6 @@
 ../../../flutter/impeller/tessellator/tessellator_unittests.cc
 ../../../flutter/impeller/toolkit/android/README.md
 ../../../flutter/impeller/toolkit/android/toolkit_android_unittests.cc
-../../../flutter/impeller/tools/check_licenses.py
 ../../../flutter/impeller/tools/malioc_cores.py
 ../../../flutter/impeller/tools/malioc_diff.py
 ../../../flutter/impeller/tools/metal_library.py