[lmi] Some redundant warning options in workhorse.make

[Vadim Zeitlin]

 Hello,

 While working on trying to make wx headers compile at maximum warning
level (you can see the warnings I've enabled for them in
https://github.com/wxWidgets/wxWidgets/pull/2033/commits/3fd55a4ddf40)
I've noticed that a couple of warnings explicitly enabled in lmi are
redundant: -Winit-self is enabled by -Wall for C++ and -Wwrite-strings is
even enabled by default in C++. As we're probably not really interested in
warnings in round_glibc.c file, which is the only non-C++ file in lmi, I
think we could remove these options from gcc_common_warnings without any
ill effects.

 Regards,
VZ

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