[flang] Remove redundant checks enforced in the grammar

Expression analysis enforces constraints implied by the grammar,
for example scalar-expr, scalar-int-expr, etc. These no longer need
to be checked during statement semantics.

Original-commit: flang-compiler/f18@35330b9a85a448453cb5db5f77697091658ddeec
Reviewed-on: https://github.com/flang-compiler/f18/pull/422
Tree-same-pre-rewrite: false