package/binutils: remove libiberty from target install

In BINUTILS_INSTALL_TARGET_CMDS (target!), libiberty is installed
to STAGING_DIR.

This is not necessary since libiberty is already unconditionally
installed to staging in BINUTILS_INSTALL_STAGING_CMDS. Furthermore
the presence of STAGING_DIR path in TARGET_CMDS is confusing and
incorrect.

Moreover libiberty is static only. Static libraries are only used at
build time, not at run time so we do not need to install libiberty in
target.

This commit removes the incorrect libiberty install in TARGET_CMDS and
adds a comment to clarify why we do not see libiberty installed on
target.

Signed-off-by: Jonathan Borne <jborne@kalray.eu>
[yann.morin.1998@free.fr:
  - reword comment, move it toward the _INSTALL_TARGET_CMDS
  - rewrap and slightly reword commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>