SourceKit: remove unncessary link against libcmark-gfm_static

There is no use of cmark in SourceKit, if there is a dependency on it
for a dependent library, we should propagate it through the dependency.

Author: compnerd

tools/SourceKit/lib/SwiftLang/CMakeLists.txt

@@ -47,7 +47,6 @@ target_link_libraries(SourceKitSwiftLang PRIVATE
   swiftOption
   swiftSymbolGraphGen
   swiftRefactoring
-  libcmark-gfm_static
   # Clang dependencies.
   clangIndex
   clangFormat