Remove $_ENV and $_SERVER access from LoggerMDC

Since the pattern layout can now provide data from the superglobal arrays, including $_SERVER and $_ENV, this functionality is no longer necessary in the MDC.

Currently you could use patterns like:
%X

{server.HTTP_HOST}

%X

{env.USER}

From 2.3 it is possible to do:
%s

{HTTP_HOST}
%e{USER}

Or better:
%server{HTTP_HOST}

%env

{USER}