Bug 31988: Remove reports/itemtypes.plugin

Bug 31988: Remove reports/itemtypes.plugin

This "plugin system" is only used for the itemtypes report. We can
simply remove the reports/manager.pl script and this plugin in favor of
a dedicated report.

Test plan:
Same behaviour expected before and after this patch

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Andrew Fuerste Henry <andrewfh@dubcolib.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 499fe0bea7d995358bd45da2bea7058d803f2b4e)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>