grep: remove unconditional fflush(stdout) from grep_printline

[bapt]

Summary

grep_printline() was flushing stdout after every output line,
causing one write(2) syscall per line. Remove this and let
stdio(3) buffer output normally. --line-buffered continues
to work via setlinebuf(3).

Benchmark on 1M lines (25MB output to file):

real time:  -65.3%  (2.17s -> 0.75s)
sys time:   -99.9%  (1.45s -> 0.00s)

--line-buffered: no significant difference.

[kevans]

Can you instead stage this as a revert of 7a7f74dbcc4f41d98218471297eaa629e34326e2, and set a periodic timer to flush, please?