Moved XL support from Gnumeric binary into real plugin.

* src/Makefile.am
(gnumeric_libs): Removed ../plugins/excel/libexcel.a

* src/main.c:
Removed:
- include "../plugins/excel/boot.h"
- xxx_debug variables
- (gnumeric_popt_options) excel debugging options
- excel_init() and excel_shutdown() calls