/etc/profile: do not add ~/bin to $PATH in system-wide init file

Users who want a user-specific bin directory to override system paths should
configure that in their user-specific ~/.bashrc, not in the system-wide init
file. The global file shouldn't add directories from user homes to $PATH
without knowing whether those actually exist or whether the users even want
them in $PATH. On my system, for example, there is no ~/bin, so I don't want my
$PATH to look for one. Removing an erroneous entry from $PATH is cumbersome,
but adding one is easy, so it feels better to err on the side of caution.

svn path=/nixos/trunk/; revision=31188