fread: remove "can be slowed down by other apps" scold message

[scottstanfield]

Not to sound jerky, cause I'm loving the fast parallel read..

...but after using fread maybe 300 times per day, I don't need to be reminded any more. A perfect tip for the FAQ, but perhaps not in production code...maybe if VERBOSE is turned on?

Here's the line of code:

data.table/src/fread.c

Line 1509 in 69ad3dc

DTPRINT("wall clock time (can be slowed down by any other open apps even if seemingly idle)\n");