Drop the useless `export` lines from `env` files.

This should fix thread safety of encoding and decoding, since
localeconv() is not tread safe after all.

Signed-off-by: lilinjie <lilinjie@uniontech.com>

Use double-backtick quoting instead. It has the same effect
(because these links had nowhere to link to) but it does not
result in loud warnings about broken references by default with Sphinx-3.x.