reftable: remove name checks

In the preceding commit we have disabled name checks in the "reftable"
backend. These checks were responsible for verifying multiple things
when writing records to the reftable stack:

  - Detecting file/directory conflicts. Starting with the preceding
    commits this is now handled by the reftable backend itself via
    `refs_verify_refname_available()`.

  - Validating refnames. This is handled by `check_refname_format()` in
    the generic ref transacton layer.

The code in the reftable library is thus not used anymore and likely to
bitrot over time. Remove it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/reftable-tests.h

@@ -14,7 +14,6 @@ int block_test_main(int argc, const char **argv);
 int merged_test_main(int argc, const char **argv);
 int pq_test_main(int argc, const char **argv);
 int record_test_main(int argc, const char **argv);
-int refname_test_main(int argc, const char **argv);
 int readwrite_test_main(int argc, const char **argv);
 int stack_test_main(int argc, const char **argv);
 int tree_test_main(int argc, const char **argv);