Revert "nvim: fix opening browser with Markdown preview on macOS + Firefox + tmux"

This reverts commit 50dfc1efa6.

It seems that the issue with open(1) was temporary and fixed by
restarting the machine.

config/nvim/lua/fschauen/plugins/markdown-preview.lua

@@ -21,17 +21,5 @@ return {
 
     -- Show preview page URL in command line when opening preview page.
     vim.g.mkdp_echo_preview_url = 1
-
-    -- Workaround for combination of macOS + Firefox + tmux:
-    -- *  On macOS, this plugin uses the open(1) command to open the preview
-    --    page in a browser.
-    -- *  For some reason, Firefox cannot be opened with open(1) from within
-    --    tmux (tested on macOS 14.5).
-    -- *  The workaround is therefore to fall back to Safari.
-    -- *  Run `open -a firefox https://schauenburg.me` from within a tmux
-    --    session -> if it works, this workaround is no longer needed.
-    if vim.fn.has("mac") and vim.env.TMUX then
-      vim.g.mkdp_browser = "safari"
-    end
   end,
 }