all: remove FIXME and TODO comments

We can always improve. These comments tend to linger and go misleading
or plain wrong over time.

Closes #16283

lib/vtls/mbedtls.c

@@ -1633,7 +1633,6 @@ static CURLcode mbedtls_sha256sum(const unsigned char *input,
                                   unsigned char *sha256sum,
                                   size_t sha256len UNUSED_PARAM)
 {
-  /* TODO: explain this for different mbedtls 2.x vs 3 version */
   (void)sha256len;
 #if MBEDTLS_VERSION_NUMBER < 0x02070000
   mbedtls_sha256(input, inputlen, sha256sum, 0);

lib/vtls/openssl.c

@@ -2049,8 +2049,6 @@ static CURLcode ossl_set_provider(struct Curl_easy *data, const char *provider)
                         sizeof(error_buffer)));
     /* Do not attempt to load it again */
     data->state.provider_failed = TRUE;
-    /* FIXME not the right error but much less fuss than creating a new
-     * public one */
     return CURLE_SSL_ENGINE_NOTFOUND;
   }
   data->state.provider = TRUE;
@@ -4283,7 +4281,6 @@ static void ossl_trace_ech_retry_configs(struct Curl_easy *data, SSL* ssl,
       servername_type = SSL_get_servername_type(ssl);
       inner = SSL_get_servername(ssl, servername_type);
       SSL_get0_ech_name_override(ssl, &outer, &out_name_len);
-      /* TODO: get the inner from BoringSSL */
       infof(data, "ECH: retry_configs for %s from %s, %d %d",
             inner ? inner : "NULL", outer ? outer : "NULL", reason, rv);
 #endif

lib/vtls/schannel.c

@@ -2367,7 +2367,6 @@ static CURLcode schannel_shutdown(struct Curl_cfilter *cf,
       Curl_pSecFn->FreeContextBuffer(outbuf.pvBuffer);
       if(!result) {
         if(written < (ssize_t)outbuf.cbBuffer) {
-          /* TODO: handle partial sends */
           failf(data, "schannel: failed to send close msg: %s"
                 " (bytes written: %zd)", curl_easy_strerror(result), written);
           result = CURLE_SEND_ERROR;

lib/vtls/vtls.c

@@ -1941,8 +1941,8 @@ CURLcode Curl_alpn_set_negotiated(struct Curl_cfilter *cf,
     else {
       *palpn = CURL_HTTP_VERSION_NONE;
       failf(data, "unsupported ALPN protocol: '%.*s'", (int)proto_len, proto);
-      /* TODO: do we want to fail this? Previous code just ignored it and
-       * some vtls backends even ignore the return code of this function. */
+      /* Previous code just ignored it and some vtls backends even ignore the
+       * return code of this function. */
       /* return CURLE_NOT_BUILT_IN; */
       goto out;
     }