[flang] Remove include of symbol.h from variable.h

This include will lead to circularities when semantics/type.h needs
include evaluate/expression.h. There will probably be a different
long-term solution.

Original-commit: flang-compiler/f18@c7fa8150368f7d3a9bb200fc5502ece0a99a8631
Reviewed-on: https://github.com/flang-compiler/f18/pull/223
Tree-same-pre-rewrite: false