ofproto-dpif-monitor: Remove unneeded calls to clear packets. #2071

Job	Run time
osx clang --disable-ssl	3m 37s
dpdk gcc	5s
linux deb no dpdk	6m 16s
linux gcc --disable-ssl	2m 8s
linux clang --disable-ssl	2m 6s
linux gcc c99	2m 56s
linux clang c99	2m 5s
linux gcc test	7m 36s
linux clang test asan	12m 37s
linux clang test ubsan	8m 53s
linux gcc test --enable-shared	10m 1s
linux clang test --enable-shared	10m 13s
linux gcc test dpdk	7m 33s
linux clang test dpdk	7m 28s
linux gcc test -ljemalloc	7m 37s
linux clang test -ljemalloc	8m 9s
linux gcc --enable-afxdp	2m 1s
linux clang --enable-afxdp	2m 3s
linux gcc dpdk --enable-shared	3m 26s
linux clang dpdk --enable-shared	2m 55s
linux gcc dpdk-shared	2m 23s
linux clang dpdk-shared	2m 13s
linux gcc dpdk-shared --enable-shared	3m 16s
linux clang dpdk-shared --enable-shared	2m 51s
linux gcc m32 --disable-ssl	2m 18s
	2h 2m 46s

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