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.

Cargo.toml

@@ -69,13 +69,13 @@ default-features = false
 features = ["suggestions", "color"]
 
 [features]
-avx-accel = ["bytecount/avx-accel", "regex/unstable"]
+avx-accel = [
+  "bytecount/avx-accel",
+]
 simd-accel = [
   "bytecount/simd-accel",
   "encoding_rs/simd-accel",
-  "regex/unstable",
 ]
-unstable = ["regex/unstable"]
 
 [profile.release]
 debug = true