Remove misleading / redundant bzero in callout_callwheel_init

The intent seems to be zeroing all of the cc_cpu array, or its singleton on
such platforms. The assumption made is that the BSP is always zero. The
code smell was introduced in r326218, which changed the prior explicit zero
to 'curcpu'. The change is only valid if curcpu continues to be zero,
contrary to the aim expressed in that commit message.

So, more succinctly, the expression could be: memset(cc_cpu,0,sizeof(cc_cpu)).

However, there's no point. cc_cpu lives in the data section and has a zero
initial value already. So this revision just removes the problematic
statement.

No functional change. Appeases a (false positive, ish) Coverity CID.

CID: 1383567
Reported by: Puneeth Jothaiah <puneethkumar.jothaia AT dell.com>
Reviewed by: kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D24089