Remove useless functions from not_implemented_yet (#237)

src/not_implemented_yet.jl

@@ -3,7 +3,4 @@
 # Some of them may have implementations in LibGEOS which we can use
 # via an extension, but there is no native-Julia implementation for them.
 
-function symdifference end
-function buffer end
-function convexhull end
-function concavehull end
+function symdifference end