lib: remove support for CURL_DOES_CONVERSIONS

TPF was the only user and support for that was dropped.

Closes #8378

include/curl/curl.h

@@ -524,10 +524,6 @@ typedef enum {
   CURLE_UPLOAD_FAILED,           /* 25 - failed upload "command" */
   CURLE_READ_ERROR,              /* 26 - couldn't open/read from file */
   CURLE_OUT_OF_MEMORY,           /* 27 */
-  /* Note: CURLE_OUT_OF_MEMORY may sometimes indicate a conversion error
-           instead of a memory allocation error if CURL_DOES_CONVERSIONS
-           is defined
-  */
   CURLE_OPERATION_TIMEDOUT,      /* 28 - the timeout time was reached */
   CURLE_OBSOLETE29,              /* 29 - NOT USED */
   CURLE_FTP_PORT_FAILED,         /* 30 - FTP PORT operation failed */
@@ -580,11 +576,7 @@ typedef enum {
   CURLE_REMOTE_FILE_EXISTS,      /* 73 - File already exists */
   CURLE_TFTP_NOSUCHUSER,         /* 74 - No such user */
   CURLE_CONV_FAILED,             /* 75 - conversion failed */
-  CURLE_CONV_REQD,               /* 76 - caller must register conversion
-                                    callbacks using curl_easy_setopt options
-                                    CURLOPT_CONV_FROM_NETWORK_FUNCTION,
-                                    CURLOPT_CONV_TO_NETWORK_FUNCTION, and
-                                    CURLOPT_CONV_FROM_UTF8_FUNCTION */
+  CURLE_OBSOLETE76,              /* 76 - NOT IN USE since 7.82.0 */
   CURLE_SSL_CACERT_BADFILE,      /* 77 - could not load CACERT file, missing
                                     or wrong format */
   CURLE_REMOTE_FILE_NOT_FOUND,   /* 78 - remote file not found */
@@ -684,6 +676,7 @@ typedef enum {
 #define CURLE_FTP_PARTIAL_FILE CURLE_PARTIAL_FILE
 #define CURLE_FTP_BAD_DOWNLOAD_RESUME CURLE_BAD_DOWNLOAD_RESUME
 #define CURLE_LDAP_INVALID_URL CURLE_OBSOLETE62
+#define CURLE_CONV_REQD CURLE_OBSOLETE76
 
 /* This was the error code 50 in 7.7.3 and a few earlier versions, this
    is no longer used by libcurl but is instead #defined here only to not