Makefile: remove check-plugin from the static checks (tikv#6887)

ref tikv#4399

Since the plugin feature is hardly used and may be very time-consuming in the Gtihub Actions CI,
this PR removes it from `make check`.

Signed-off-by: JmPotato <ghzpotato@gmail.com>

Makefile

@@ -143,7 +143,7 @@ install-tools:
 
 #### Static checks ####
 
-check: install-tools tidy static generate-errdoc check-plugin check-test
+check: install-tools tidy static generate-errdoc check-test
 
 static: install-tools
 	@ echo "gofmt ..."