debug/gdb: cleanup (remove) the static ncurses build

ncurses is built solely for the sake of building a native gdb.
The user should not rely on this library to build his/her userland,
but should rather build his/her own. So we remove it from the
sysroot after we successfully build the native gdb.

 # x86 specific options

 config ARCH_x86

     select ARCH_USE_MMU

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_CPU

     select ARCH_SUPPORT_TUNE

     help

       The x86 architecture, as defined by:

         http://www.intel.com/