Switch simd feature back to opt-in, it can be flipped in a subsequent PR

Author: Shnatsel

Cargo.toml

@@ -12,7 +12,7 @@ repository = "https://github.com/vstroebel/jpeg-encoder"
 rust-version = "1.87"
 
 [features]
-default = ["std", "simd"]
+default = ["std"]
 simd = ["std"]
 std = []