rtc_linux.c: Remove useless assignment `error = -1;`

rtc_linux.c: Remove useless assignment `error = -1;`

The Clang static analyzer scan-build from Debian clang version 3.4-1
found the following unneeded assignment.

        rtc_linux.c:756:5: warning: Value stored to 'error' is never read
            error = 1;
            ^       ~

Indeed, if in that if branch, the function returns without ever looking
at the variable `error`. So remove the line.