Makefile: remove stale references to UNIT_TEST_PROGRAMS

As of 9340e7c (t/unit-tests: finalize migration of reftable-related
tests, 2025-06-05), all of the `UNIT_TEST_PROGRAMS` logic has moved from
`Makefile` to `t/Makefile`.

This commit removes no longer necessary references.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

Makefile

@@ -697,7 +697,6 @@ TEST_BUILTINS_OBJS =
 TEST_OBJS =
 TEST_PROGRAMS_NEED_X =
 THIRD_PARTY_SOURCES =
-UNIT_TEST_PROGRAMS =
 UNIT_TEST_DIR = t/unit-tests
 UNIT_TEST_BIN = $(UNIT_TEST_DIR)/bin
 
@@ -2829,7 +2828,6 @@ OBJECTS += $(FUZZ_OBJS)
 OBJECTS += $(REFTABLE_OBJS) $(REFTABLE_TEST_OBJS)
 OBJECTS += $(UNIT_TEST_OBJS)
 OBJECTS += $(CLAR_TEST_OBJS)
-OBJECTS += $(patsubst %,$(UNIT_TEST_DIR)/%.o,$(UNIT_TEST_PROGRAMS))
 
 ifdef INCLUDE_LIBGIT_RS
 	OBJECTS += contrib/libgit-sys/public_symbol_export.o