string_view: Drop constexpr constructor for standard compatibility

string_view: Drop constexpr constructor for standard compatibility

APT::StringView is supposed to be a temporary measure, until support
for the standardized string_view is widely available. Introducing
additional unstandardized features just makes porting to the
standard version harder.

The constexpr constructor also won't have any real effect on most
systems, as the compiler will happily optimise the strlen() call
away for constant strings.

Gbp-Dch: ignore