Don't fsync() in checksum

This gives a major boost in librepo performance. For a reposync of an
Amazon Linux 2023 x86-64 repository on a m5n.16xlarge EC2 instance with
a 500MB/sec 3000IOP EBS volume, this alone reduces run time by 30
seconds of wall time, and gets reposync nearly using a whole core rather
than only two thirds of one.

librepo/checksum.c

@@ -266,17 +266,6 @@ lr_checksum_fd_compare(LrChecksumType type,
 
     *matches = (strcmp(expected, checksum)) ? FALSE : TRUE;
 
-    if (fsync(fd) != 0) {
-        if (errno == EROFS || errno == EINVAL) {
-            g_debug("fsync failed: %s", strerror(errno));
-        } else {
-            g_set_error(err, LR_CHECKSUM_ERROR, LRE_FILE,
-                        "fsync failed: %s", strerror(errno));
-            lr_free(checksum);
-            return FALSE;
-        }
-    }
-
     if (caching && *matches && timestamp != -1) {
         // Store timestamp and checksum as extended file attribute if caching is enabled
         FSETXATTR(fd, timestamp_key, timestamp_str, strlen(timestamp_str), 0);