Don't strip invisible text when formatting hover string

Don't strip invisible text when formatting hover string

This was introduced in https://github.com/joaotavora/eglot/issues/482 due to a bad interaction with a specific
server.  But this solution makes hyperlinks in Eldoc buffers
unclickable, because the markdown-mode function that visits a link
relies on the invisible text.

Per https://github.com/joaotavora/eglot/issues/866

* eglot.el (eglot--format-markup): Use buffer-string instead of
filter-buffer-substring

GitHub-reference: fix https://github.com/joaotavora/eglot/issues/865