Remove textDocument/diagnostic capability (#13102)

Zed currently does not support pull diagnostics, yet still has the
capability for it (`textDocument/diagnostic`) (added in
https://github.com/zed-industries/zed/commit/14993e087669c7103df2239a2628ee0d3ce38596).
Some language servers therefore assume Zed will use pull diagnostics,
which leads to there being no diagnostics at all. This PR removes this
capability, making it possible to get diagnostics with more language
servers.

Release Notes:

- N/A