clang-tidy: check readability-redundant-preprocessor, fix fallouts

Also:
- cipher_suite: merge `USE_MBEDTLS` `#if` blocks.

Ref: https://clang.llvm.org/extra/clang-tidy/checks/readability/redundant-preprocessor.html

Closes #20628

lib/content_encoding.c

@@ -600,9 +600,7 @@ static const struct Curl_cwtype * const general_unencoders[] = {
 
 /* supported content decoders only for transfer encodings */
 static const struct Curl_cwtype * const transfer_unencoders[] = {
-#ifndef CURL_DISABLE_HTTP
   &Curl_httpchunk_unencoder,
-#endif
   NULL
 };