package/bctoolbox: drop GIT_EXECUTABLE

package/bctoolbox: drop GIT_EXECUTABLE

GIT_EXECUTABLE is not needed since version 4.3.0 and
https://github.com/BelledonneCommunications/bctoolbox/commit/a92ea8672fc0a736c9018de31588aeeeef4a4157
https://github.com/BelledonneCommunications/bctoolbox/commit/6c2e02ffb16f999e270c5de29bbf4dd13b9e986d

CMake Warning:
  Manually-specified variables were not used by the project:

    BUILD_DOC
    BUILD_DOCS
    BUILD_EXAMPLE
    BUILD_EXAMPLES
    BUILD_TEST
    BUILD_TESTING
    BUILD_TESTS
    GIT_EXECUTABLE

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>