internal/lsp: stop providing insertText for completion items
internal/lsp: stop providing insertText for completion items

InsertText is deprecated, and it seems that providing both InsertText
and TextEdits causes unexpected behavior from VSCode. Avoid this by
providing only TextEdits.

Fixes golang/go#30796

Change-Id: Ieb5ad2fecd6f7083a4c1bc402634893c7e6ff49f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/167457
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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