Bug 27246: Remove obsolete BEGIN code from C4::Context

Bug 27246: Remove obsolete BEGIN code from C4::Context

We don't longer enter the HTTP_USER_AGENT conditional in the BEGIN block
of C4::Context. It was used when apache only was used.

I am suggesting to remove it but keep the $CGI::VERSION < 4.08 condition
to keep support of all version (not sure it's needed but won't hurt).

I have not found a lot of occurrences of KOHA_BACKTRACES and I am
assuming not much people are using it, we should prefer the dev_install
flag instead (to set environment="development" when plack is started and
enabled the starman trace)

Test plan:
Regression test what you think is useful
Confirm the above statements

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>