mkcompile_h: git rid of UTS_TRUNCATE from LINUX_COMPILE_{BY,HOST}

UTS_VERSION is set to struct uts_namespace, hence a too long string
should be truncated so it fits in 64 characters.

On the other hand, LINUX_COMPILE_BY/HOST are not set to uts_namespace.
They are just used in the banners, which do not have specific length
limitation.

I dug into the git history, but I could not find the reason why
these two strings must fit in 64 characters. Remove them.

Now that UTS_VERSION is the only user of UTS_TRUNCATE, I squashed it.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>