Fix loss of reference highlights on buffer change

[stephan-t]

This PR fixes an issue where document highlighting of references will stop working on buffers that have switched to a different buffer within the same window.

Instead of returning after checking if a buffer already has a text property type added, this commit will remove the existing property type and add it back each time a buffer changes.

To reproduce the issue:

• enable g:lsp_document_highlight_enabled
• open a file supported by a language server
• highlight a reference
• switch to another buffer (e.g., :bn, :e new, :LspDefinition)
• switch back to previous buffer (e.g., :bp, CTRL+6, CTRL+O)
• try to highlight a reference

Before fix:

After fix: