gdb: drop duplicate dependencies

gdb: drop duplicate dependencies

Commit 1e3738db42919cb4487f7a64381ac7d3090d0e1f ("package/gdb: rework
dependency for C++11") erroneously added duplicate dependencies in
package/gdb/Config.in related to gcc 4.8 and C++ dependency.

One copy of those dependencies was below the select causing a
check-package warning.

Fixes:

package/gdb/Config.in:28: attributes order: type, default, depends on, select, help (http://nightly.buildroot.org/#_config_files)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>