[gdb/testsuite] Remove invalid .debug_line section in gdb.dwarf2/dw2-icycle.exp

Test-case source gdb.dwarf2/dw2-icycle.S contains an invalid .debug_line
section contribution, causing a readelf -w warning.

It's also unused, so fix the warning by removing it.

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

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34157