deps: get rid of unstable feature

This was introduced as a temporary measure for dealing with the regex
crate's unstable feature, but it was never included in a release of
ripgrep. Thus, we remove it. The regex crate will now automatically enable
SIMD optimizations when available.

appveyor.yml

@@ -50,7 +50,7 @@ test_script:
 
 before_deploy:
   # Generate artifacts for release
-  - cargo build --release --features unstable
+  - cargo build --release
   - mkdir staging
   - copy target\release\rg.exe staging
   - ps: copy target\release\build\ripgrep-*\out\_rg.ps1 staging