trace.c, git.c: remove unnecessary parameter to trace_repo_setup()

trace.c, git.c: remove unnecessary parameter to trace_repo_setup()

trace_repo_setup() of trace.c is called with the argument 'prefix' from
only one location, run_builtin of git.c, which sets 'prefix' to the return
value of setup_git_directory() or setup_git_directory_gently() (a wrapper
of the former).

Now that "prefix" is in startup_info there is no need for the parameter
of trace_repo_setup() because setup_git_directory() sets "startup_info->prefix"
to the same value it returns. It would be less confusing to use "prefix"
from startup_info instead of passing it as an argument.

Signed-off-by: Idriss Fekir <mcsm224@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>