Remove an unnecessary overload from ASTLambda.h 
As Richard pointed out to me, dyn_cast is very cheap - there is no real benefit from adding cluttery overloads to only avoid that cast.
No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191646 91177308-0d34-0410-b5e6-96231b3b80d8