build: remove FALLTHROUGH_INTENDED from leveldb.cmake

No-longer needed after
https://github.com/bitcoin-core/leveldb-subtree/pull/60.

cmake/leveldb.cmake

@@ -56,7 +56,6 @@ target_compile_definitions(leveldb
     HAVE_FDATASYNC=$<BOOL:${HAVE_FDATASYNC}>
     HAVE_FULLFSYNC=$<BOOL:${HAVE_FULLFSYNC}>
     HAVE_O_CLOEXEC=$<BOOL:${HAVE_O_CLOEXEC}>
-    FALLTHROUGH_INTENDED=[[fallthrough]]
     $<$<NOT:$<BOOL:${WIN32}>>:LEVELDB_PLATFORM_POSIX>
     $<$<BOOL:${WIN32}>:LEVELDB_PLATFORM_WINDOWS>
     $<$<BOOL:${WIN32}>:_UNICODE;UNICODE>