NaCl docs: Remove outdated note about GDB debugging prerequisites

Chrome 23 was released about a year ago, so saying that SDK version 23
is required now seems unnecessary.

There is no general requirement to use a Chrome version that's as new
as the SDK version.  This note was left over from last year when the
NaCl GDB debug stub was undergoing a lot of development, and nacl-gdb
tended to require new debug stub features in Chrome, but there have
been no such dependencies added recently.

BUG=none
TEST="make" in native_client_sdk/src/doc

Review URL: https://codereview.chromium.org/72763003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235168 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235168