internal/lsp: fix infinite recursion in CancelRequest
internal/lsp: fix infinite recursion in CancelRequest

Quick fix to (*server).CancelRequest, but we haven't implemented actual
support for it. Filed golang/go#35679 to track this.

Change-Id: Ic0de01d49b779c4f0656587584fbd2bf8791d0ce
Reviewed-on: https://go-review.googlesource.com/c/tools/+/207718
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Reviewed-by: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools