Remove plain text file type association from default settings (#25420)

Closes #20291

This PR removes the plain text file association from the default
settings, as #21298 added a `LanguageMatcher` for Plain Text files,
which now associates "Plain Text" with `txt`-files (see
10053e2566/crates/language/src/language.rs (L127-L137)).

Thus, the association via the default settings is not required anymore,
which fixes #20291 as described in
https://github.com/zed-industries/zed/issues/20291#issuecomment-2500731743

Release Notes:

- Fixed default file type associations overriding associations provided
by extensions for `txt`-files.

Co-authored-by: Peter Tripp <peter@zed.dev>

assets/settings/default.json

@@ -1055,7 +1055,6 @@
   // }
   //
   "file_types": {
-    "Plain Text": ["txt"],
     "JSONC": ["**/.zed/**/*.json", "**/zed/**/*.json", "**/Zed/**/*.json", "**/.vscode/**/*.json"],
     "Shell Script": [".env.*"]
   },