Use make's builtin RM variable

At least as far as removing individual files goes, this is the best
pattern for removing. I can't say the same for removing directories, but
I went ahead and changed those to `$(RM) -r` anyway.

Signed-off-by: Tristan Partin <tristan@neon.tech>

Makefile

@@ -297,7 +297,7 @@ clean: postgres-clean neon-pg-clean-ext
 # This removes everything
 .PHONY: distclean
 distclean:
-	rm -rf $(POSTGRES_INSTALL_DIR)
+	$(RM) -r $(POSTGRES_INSTALL_DIR)
 	$(CARGO_CMD_PREFIX) cargo clean
 
 .PHONY: fmt
@@ -329,7 +329,7 @@ postgres-%-pgindent: postgres-%-pg-bsd-indent postgres-%-typedefs.list
 		$(ROOT_PROJECT_DIR)/vendor/postgres-$*/src/tools/pgindent/pgindent --typedefs postgres-$*-typedefs-full.list \
 		$(ROOT_PROJECT_DIR)/vendor/postgres-$*/src/ \
 		--excludes $(ROOT_PROJECT_DIR)/vendor/postgres-$*/src/tools/pgindent/exclude_file_patterns
-	rm -f pg*.BAK
+	$(RM) pg*.BAK
 
 # Indent pxgn/neon.
 .PHONY: neon-pgindent

compute/Makefile

@@ -34,7 +34,7 @@ sql_exporter_autoscaling.yml: $(jsonnet_files)
 
 .PHONY: clean
 clean:
-	rm -f \
+	$(RM) \
 		etc/neon_collector.yml \
 		etc/neon_collector_autoscaling.yml \
 		etc/sql_exporter.yml \

pgxn/neon/Makefile

@@ -54,7 +54,7 @@ walproposer-lib: libwalproposer.a;
 
 .PHONY: libwalproposer.a
 libwalproposer.a: $(WALPROP_OBJS)
-	rm -f $@
+	$(RM) $@
 	$(AR) $(AROPT) $@ $^
 
 # needs vars: