[clangd] Remove the recovery-ast options.

These force a couple of flags or that are now on by default.
So the flags don't currently do anything unless the compile command has
-fno-recovery-ast explicitly.

(For turning recovery *off* for debugging we can inject the flag with config)

This leaves the command-line flags around with no effect, I'm planning to add
a "retired flag" mechanism shortly in a separate patch.

Differential Revision: https://reviews.llvm.org/D94724