Drop unneeded TEST_SHELL_PATH make variable
    
    TEST_SHELL_PATH was added in 62f562d ("Use 'prove' as test harness,
    enable shell tracing", 2018-01-18).  Shortly afterward, upstream commit
    a5bf824f3b ("t: prevent '-x' tracing from interfering with test helpers'
    stderr", 2018-02-25)¹ removed the need to use bash to get the benefits of
    running the test suite with '-x' tracing.
    
    ¹ https://github.com/git/git/commit/a5bf824f3b