Remove registration of the legacy LanguageHandler class

[rwols]

I can't find any package any more that still uses this.

[predragnikolic]

I guess only these 2 plugins that are published on PC still use LanguageHandler:

https://github.com/prometheus-community/sublimelsp-promql/blob/master/plugin.py#L130 (not part of the LSPSublimeOrg)
https://github.com/sublimelsp/LSP-Serenata/blob/master/plugin.py#L10

[predragnikolic]

Will you also delete the /LSP/plugin/core/handlers.py file?

[jfcherng]

I guess only these 2 plugins that are published on PC still use LanguageHandler:

prometheus-community/sublimelsp-promql@master/plugin.py#L130 (not part of the LSPSublimeOrg) sublimelsp/LSP-Serenata@master/plugin.py#L10

LSP-Serenata hasn't updated for 2 years (still using an old server) and the server, Serenata was last active commit in 10 months ago. Consider PHP is evolving every year, I may assume both of LSP-Serenata and Serenata are gradually outdated...

[rchl]

In boot.py those are unused:

• import weakref
• Callable and Tuple from .plugin.core.typing

Also maybe we should be defining __all__ to clearly indicate which imports are "public". That would also get rid of all false positive warnings that import is "unused".