[clangd] Remove `allCommitCharacters`

This was added in 2a095ff6, however it never worked with VSCode
due to bugs in vscode-languageclient
(https://github.com/microsoft/vscode-languageserver-node/issues/673).
Now that it does work, we can tell the interactions with text editing, with
snippets, and vscode's select-first-completion behavior are bad.

The spec is vague and clients could do something reasonable with the
current values. However they could clearly do something unreasonable
too, and over time behavior+spec tends to converge on VSCode's behavior.

This addresses https://github.com/clangd/vscode-clangd/pull/358
See also https://github.com/clangd/vscode-clangd/pull/358 which hotfixes
this on the client-side (as we can't apply this change retroactively to
clangd 12-14).

Differential Revision: https://reviews.llvm.org/D129579