lib: Eliminate dead code

lib: Eliminate dead code

The tz function is only called if ENV_TZ starts with a slash.

If the specified file cannot be read, the code implies that ENV_TZ
would be returned if it does not start with a slash.

Since we know that it DOES start with a slash, the code can be
simplified to state that "TZ=CST6CDT" is returned as a default if
the specified file cannot be read.

Benefit of this change is that strcpy's use case here can be
easier verified.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>