gitweb.js: No need for inProgress in blame_incremental.js

JavaScript is single-threaded, so there is no need for protection
against re-entrancy via inProgress variable.

In particular calls to setInterval handler are stacked if handler
doesn't finish before new interrupt (before new interval).  The same
happens with events - they are (hopefully) stacked if even handler
didn't finish work.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>