msgfmt: Remove POT-Creation-Date field from the header in the output.

msgfmt: Remove POT-Creation-Date field from the header in the output.

This helps reproducible builds.
Reported at <https://savannah.gnu.org/bugs/?49654>.
* gettext-tools/src/msgl-header.h (message_list_delete_header_field): New
declaration.
* gettext-tools/src/msgl-header.c (known_fields): New variable, extracted from
msgdomain_list_set_header_field.
(message_list_delete_header_field): New function.
* gettext-tools/src/write-mo.c: Include msgl-header.h.
(msgdomain_write_mo): Delete the POT-Creation-Date field.
* gettext-tools/src/write-java.c: Include msgl-header.h.
(msgdomain_write_java): Delete the POT-Creation-Date field.
* gettext-tools/src/write-csharp.c: Include msgl-header.h.
(msgdomain_write_csharp): Delete the POT-Creation-Date field.
* gettext-tools/src/write-resources.c: Include msgl-header.h.
(msgdomain_write_csharp_resources): Delete the POT-Creation-Date field.
* gettext-tools/src/write-tcl.c: Include msgl-header.h.
(msgdomain_write_tcl): Delete the POT-Creation-Date field.
* gettext-tools/src/write-qt.c: Include msgl-header.h.
(msgdomain_write_qt): Delete the POT-Creation-Date field.
* gettext-tools/src/write-desktop.c: Include msgl-header.h.
(msgdomain_write_desktop): Delete the POT-Creation-Date field.
* gettext-tools/src/write-xml.c: Include msgl-header.h.
(msgdomain_write_xml): Delete the POT-Creation-Date field.
* gettext-tools/tests/msgfmt-19: New file, based on
gettext-tools/tests/msgfmt-18.
* gettext-tools/tests/Makefile.am (TESTS): Add it.