Remove ~ and P maps from blame documentation

These maps are not nearly as useful as they seem.  Let's avoid tempting
newcomers to use them.  I recommend to instead use <CR>, which shows the
change in context.

References: https://github.com/tpope/vim-fugitive/issues/2070

doc/fugitive.txt

@@ -70,8 +70,6 @@ that are part of Git repositories).
                         O     jump to patch or blob in new tab
                         p     jump to patch or blob in preview window
                         -     reblame at commit
-                        ~     reblame at commit~[count]
-                        P     reblame at commit^[count]
 
                                                 *g:fugitive_dynamic_colors*
                         In the GUI or a 256 color terminal, commit hashes will