libutee: remove Trusted UI code

Removes all the TUI-related code from libutee (lib/libutee/tui), as
well as its  dependencies: lib/libpng and lib/libzlib. Two reasons for
this:
1. This is far from being a complete and testable TUI implementation.
In other words, it is dead code, more or less.
2. lib/libzlib (version 1.2.8) contains several CVE vulnerabilities.
Even if the code is not used, it may trigger some code analysis tools
and is a problem for some projects.

Reported-by: Jianhui Li <airbak.li@hisilicon.com>
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>

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