warnless: delete orphan declarations

[vszakats]

Follow-up to 358f7e7 #11932
Closes #13639

---

These functions had a point not just for the Intel compiler but for envs that don't compile FD_SET()/FS_ISSET() cleanly with -Wsign-conversion. E.g. OmniOS and older Cygwin.
Ref: #13489
Ref:

curl/docs/examples/sendrecv.c

Lines 51 to 65 in d84a95d

	#if defined(__GNUC__)
	#pragma GCC diagnostic push
	#pragma GCC diagnostic ignored "-Wsign-conversion"
	#endif
	FD_SET(sockfd, &errfd); /* always check for error */
	if(for_recv) {
	FD_SET(sockfd, &infd);
	}
	else {
	FD_SET(sockfd, &outfd);
	}
	#if defined(__GNUC__)
	#pragma GCC diagnostic pop
	#endif