[clang] Remove `-triple` from the invocations of `flang-new -fc1`

[clang] Remove `-triple` from the invocations of `flang-new -fc1`

This is just a small change in the Flang tool within libclangDriver.
Currently it passes `-triple` when calling `flang-new -fc1` for various
driver Jobs. As there is no support for code-generation, `-triple` is
not required and should remain unsupported. It is safe to remove it.

This hasn't been a problem as the affected driver Jobs are not yet
implemented or used. However, we will be adding support for them in the
near future and the fact `-triple` is added will become a problem.

Differential Revision: https://reviews.llvm.org/D93027