ofproto-dpif-monitor: Remove unneeded calls to clear packets. #2073

Job	Run time
osx clang --disable-ssl	2m 47s
linux gcc --disable-ssl	2m 0s
linux clang --disable-ssl	2m 3s
linux gcc c99	2m 6s
linux clang c99	2m 11s
linux gcc test 4.14	7m 40s
linux clang test 4.14 asan	12m 58s
linux gcc test --enable-shared	9m 49s
linux clang test --enable-shared	9m 48s
linux gcc test dpdk	7m 51s
linux clang test dpdk	7m 43s
linux gcc test -ljemalloc	7m 20s
linux clang test -ljemalloc	9m 47s
linux gcc 5.8 5.5 5.4 4.19	5m 38s
linux clang 5.8 5.5 5.4 4.19	5m 42s
linux gcc 4.14 4.9 4.4	3m 38s
linux clang 4.14 4.9 4.4	3m 59s
linux gcc afxdp 5.4	2m 56s
linux clang afxdp 5.4	3m 6s
linux gcc dpdk --enable-shared	2m 57s
linux clang dpdk --enable-shared	2m 59s
linux gcc dpdk-shared	2m 29s
linux clang dpdk-shared	2m 30s
linux gcc dpdk-shared --enable-shared	2m 55s
linux clang dpdk-shared --enable-shared	2m 51s
linux gcc m32 --disable-ssl	2m 1s
linux gcc deb	6m 48s
	2h 14m 32s

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