Removing string as a token.

- The string keyword is reserved for future use but should not be a
  token in the code since it can cause trigger internal compiler
  assertions.

- fixes #1384

ExpressionCompiler.h

@@ -42,7 +42,6 @@ class CompilerContext;
 class Type;
 class IntegerType;
 class ArrayType;
-class StaticStringType;
 
 /**
  * Compiler for expressions, i.e. converts an AST tree whose root is an Expression into a stream

Token.h

@@ -290,7 +290,6 @@ namespace solidity
 	K(Byte, "byte", 0)                                                 \
 	K(Address, "address", 0)                                           \
 	K(Bool, "bool", 0)                                                 \
-	K(StringType, "string", 0)                                         \
 	K(Real, "real", 0)                                                 \
 	K(UReal, "ureal", 0)                                               \
 	T(TypesEnd, NULL, 0) /* used as type enum end marker */            \