buildhistory: Drop BUILDHISTORY_RESET due to reliability issuesHEADmaster

The implementation of BUILDHISTORY_RESET is problematic, particlarly given that
people are trying to create an API with it alongside BUILDHISTORY_PRESERVE
which simply doesn't exist and can't work reliably. Worse, the code paths with
this bolted on implementation are convoluted and near impossible to follow.

BUILDHISTORY_PRESERVE is effectively internal API, used to stop buildhistory
removing some files which are needed for data, or are created at different
parts of the build. Add a comment to explain what it is doing and why these files
are listed.

Commit 9f68a45aa238ae5fcdfaca71ba0e7015e9cb720e tried to "fix" preserve support
with the reset functionality but it didn't fully work and has just exposed futher
issues. There is a further fix however I can brely follow the code and in reviewing
it, I've concluded we shouldn't be doing this at all.

Due to the way BUILDHISTORY_RESET was implemented, horrible races were introduced
making it unclear what happens to the data if builds fail for example, or how sstate
interacts with the build since things get reset but stamps do not and tasks may not
rerun. It also interacts badly with any additions to the preserve list, due to
misunderstandings on what that variable does.

Having stared long and hard at the code, and really struggled to understand it, I',
of the view that "reset" for CI purposes should be done by the CI itself. The CI can
choose to remove some files or all files and decide how to handle failures. It has
to handle the buildhistory directory anyway.

Therefore drop BUILDHISTORY_RESET support, allowing the "old" codepaths to be dropped.
BUILDHISTORY_PRESERVE is better documented to hint that it is internal API and to
show what it is really for.

If we really do want some functionality list this, it needs to be implemented in a
way you can follow the code, and have tests.

(From OE-Core rev: 15c5258fd0063ace425b7e904521f1695ffb2a85)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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