[docs] Remove unmaintained target feature matrix

Back in 2017, a table was added to the codegen documentation listing
which features various backends support. It received a few updates since
then, but not since the end of 2019. Having such a table is a nice idea,
but it hasn't been kept up to date, it isn't easy to ensure that it is
up to date, and the table probably isn't very discoverable for most
users who would be interested in this information anyway (it would be
better suited to some kind of "what can LLVM do for me?" page).

For all of the above reasons, I believe it makes sense to remove it.

Differential Revision: https://reviews.llvm.org/D129996

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