Remove default max concurrent compaction limit

PR #9204 introduced a maximum default concurrent compaction limit of 4.
The idea was to reduce IO utilisation on large systems with many cores,
and high write load. Often on these systems, disks were not scaled
appropriately to to the write volume, and while the write path could
keep up, compactions would saturate disks.

In #9225 work was done to reduce IO saturation by limiting the
compaction throughput. To some extent, both #9204 and #9225 work towards
solving the same problem.

We have recently begun to notice larger clusters to suffer from
situations where compactions are not keeping up because they have been
scaled up, but the limit of 4 has stayed in place. While users can
manually override the setting, it seems more user friendly if we remove
the limit by default, and set it manually in cases where compactions are
causing too much IO on large boxes.

tsdb/store.go

@@ -217,11 +217,6 @@ func (s *Store) loadShards() error {
 	if lim == 0 {
 		lim = runtime.GOMAXPROCS(0) / 2 // Default to 50% of cores for compactions
 
-		// On systems with more cores, cap at 4 to reduce disk utilization
-		if lim > 4 {
-			lim = 4
-		}
-
 		if lim < 1 {
 			lim = 1
 		}