Don't GC unprotect hash in Coverage.peek_result

The call to unprotect the coverages hash was introduced in commit 99093e1
where we used the raw ST table and st_foreach. We now use rb_hash_foreach
which no longer requires GC unprotecting the coverages hash.

Author: peterzhu2118

ext/coverage/coverage.c

@@ -352,7 +352,6 @@ rb_coverage_peek_result(VALUE klass)
     if (!RTEST(coverages)) {
         rb_raise(rb_eRuntimeError, "coverage measurement is not enabled");
     }
-    OBJ_WB_UNPROTECT(coverages);
 
     rb_hash_foreach(coverages, coverage_peek_result_i, ncoverages);