gdb/objfiles: use filtered_iterator as objfile::section_iterator

gdb/objfiles: use filtered_iterator as objfile::section_iterator

objfile::section_iterator looks like a good candidate to be implemented
with filtered_iterator.  Following the enhancements to filtered_iterator
in the previous patch, it's pretty straighforward.

I removed the non-const version of objfile::sections, because it didn't
seem useful to have the two methods returning the exact same type and
value.  Having just the const version achieves the same thing.

Change-Id: I2f29c2fb3f95605cb816cc1ff8935c10e0496052
Approved-By: Tom Tromey <tom@tromey.com>