selftests: kvm: remove meaningless assignments in Makefiles

$(shell ...) expands to the output of the command. It expands to the
empty string when the command does not print anything to stdout.
Hence, $(shell mkdir ...) is sufficient and does not need any
variable assignment in front of it.

Commit c2bd08ba ("treewide: remove meaningless assignments in
Makefiles", 2024-02-23) did this to all of tools/ but ignored in-flight
changes to tools/testing/selftests/kvm/Makefile, so reapply the change.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/Makefile

@@ -277,7 +277,7 @@ TEST_DEP_FILES += $(patsubst %.o, %.d, $(LIBKVM_OBJS))
 TEST_DEP_FILES += $(patsubst %.o, %.d, $(SPLIT_TEST_GEN_OBJ))
 -include $(TEST_DEP_FILES)
 
-x := $(shell mkdir -p $(sort $(OUTPUT)/$(ARCH_DIR) $(dir $(LIBKVM_C_OBJ) $(LIBKVM_S_OBJ))))
+$(shell mkdir -p $(sort $(OUTPUT)/$(ARCH_DIR) $(dir $(LIBKVM_C_OBJ) $(LIBKVM_S_OBJ))))
 
 $(filter-out $(SPLIT_TEST_GEN_PROGS), $(TEST_GEN_PROGS)) \
 $(TEST_GEN_PROGS_EXTENDED): %: %.o
@@ -309,7 +309,7 @@ $(LIBKVM_S_OBJ): $(OUTPUT)/%.o: %.S $(GEN_HDRS)
 $(LIBKVM_STRING_OBJ): $(OUTPUT)/%.o: %.c
 	$(CC) $(CFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c -ffreestanding $< -o $@
 
-x := $(shell mkdir -p $(sort $(dir $(TEST_GEN_PROGS))))
+$(shell mkdir -p $(sort $(dir $(TEST_GEN_PROGS))))
 $(SPLIT_TEST_GEN_OBJ): $(GEN_HDRS)
 $(TEST_GEN_PROGS): $(LIBKVM_OBJS)
 $(TEST_GEN_PROGS_EXTENDED): $(LIBKVM_OBJS)