[MIPS] remove Documentation/mips/GT64120.README

Based upon the 2.4 kernel, the information presented in the
Documentation/mips/GT64120.README file is outdated. Worse,
the document contents are plain misleading nowadays because
the text mentions files and directories, which have been
deleted, moved or restructured for 2.6.

This patch removes the documentation, which is no more valid.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>