closes bpo-45479: Degunkify Py_UniversalNewlineFgets.

[benjaminp]

Remove dead variables and control flow.

https://bugs.python.org/issue45479

[eryksun]

This block never executes. skipnextlf is only set if c == '\r' in the next block, in which case c is set to '\n', and the execution flow breaks out of the loop via if (c == '\n') break. The value of skipnextlf is handled after the loop. The loop can be simplified as follows:

    while (--n > 0 && (c = GETC(stream)) != EOF) {
        if (c == '\r') {
            /* Translate \r to \n, and skip an adjacent \n, if any. */
            skipnextlf = 1;
            c = '\n';
        }
        *p++ = c;
        if (c == '\n') {
            break;
        }
    }