build: unset variables ending with "URL" if USE_EXISTING_SRC_DIR is ON

This will reduce required boilerplate, but more importantly it will
automatically unset variables ending on URL. This will help people
needing to avoid to unset the URL variable each time a new dependency is
added.

It is possible that this may remove a non-download variable ending on
"URL" in the future, however, the risk of this is likely much lower than
the risk of someone forgetting to unset the variable.

cmake.deps/cmake/BuildTreesitterParsers.cmake

@@ -1,8 +1,5 @@
 function(BuildTSParser LANG TS_URL TS_SHA256 TS_CMAKE_FILE)
   set(NAME treesitter-${LANG})
-  if(USE_EXISTING_SRC_DIR)
-    unset(TS_URL)
-  endif()
   ExternalProject_Add(${NAME}
     URL ${TS_URL}
     URL_HASH SHA256=${TS_SHA256}