http2: remove unused Curl_http2_strerror function declaration

Curl_http2_strerror was renamed to http2_strerror in
05b100aee2 and then http2_strerror was removed in
5808a0d0f5

This also fixes the following compiler error

lib/http2.h:41:33: error: unknown type name 'uint32_t'
lib/http2.h:1:1: note: 'uint32_t' is defined in header '<stdint.h>'

Closes #10912

lib/http2.h

@@ -38,8 +38,6 @@
  */
 void Curl_http2_ver(char *p, size_t len);
 
-const char *Curl_http2_strerror(uint32_t err);
-
 CURLcode Curl_http2_request_upgrade(struct dynbuf *req,
                                     struct Curl_easy *data);