Keybindings hints should respect unicode setting

[jose1711]

Even with use_unicode_icons set to No calcure tries to use display left/right arrows: 🠒 🠐. With fonts missing unicode support this may lead to:

[anufrievroman]

Well, since these strings are parts of the translation files it's easier just to delete these symbols from help screen and just keep n and p instead of trying to somehow take that option into account.

In the documentation users can see full list of key bindings, which also includes h,j,k,l and arrow keys.

[jose1711]

or perhaps this would be an option:

n,<-  Next month (day)
p,->  Previous month (day)

[anufrievroman]

I'll just mention the arrow keys together with Vim keys at the bottom of the help screen.
Changed in 3117a8c