tests/plugin: Remove duplicate insn log from libinsn.so

tests/plugin: Remove duplicate insn log from libinsn.so

This is a perfectly natural occurrence for x86 "rep movb",
where the "rep" prefix forms a counted loop of the one insn.

During the tests/tcg/multiarch/memory test, this logging is
triggered over 350000 times.  Within the context of cross-i386-tci
build, which is already slow by nature, the logging is sufficient
to push the test into timeout.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>