reproducible_build: Remove BUILD_REPRODUCIBLE_BINARIES checking

Previously if BUILD_REPRODUCIBLE_BINARIES was set to 0, the system would
fall back and select the default epoch (April 2011), but still perform
the reproducible build actions.  This resulted in binaries that had an
unusually old date.

Simplify the functions and remove the anonymous python as no longer
necessary.

Also improve the documentation to better explain what the class is doing
and how a recipe can override the behavior if necessary.

Signed-off-by: Mark Hatle <mark.hatle@xilinx.com>
Signed-off-by: Mark Hatle <mark.hatle@kernel.crashing.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>