Remove deprecated syntax for repeating single keyword

[pekkaklarck]

We still support very old syntax for repeating a single keyword:

*** Test Cases ***
Example
    3x    Log    This is run three times

Repeating a single keyword is not that often needed in general and having special syntax for it is absolutely not worth it. It has been possible to use Repeat Keyword if repeating is actually needed since RF 2.0.4 (#192). This requires no extra syntax and is also more explicit:

*** Test Cases ***
Example
    Repeat Keyword    3x    Log    This is run three times

The old syntax for repeating a single keyword was originally deprecated in RF 2.0.4 (#193) and removed in RF 2.5 (#194), but it was added back in RF 2.5.5 (#731) because removing caused problems to some users. Deprecation warning was kept and it ought to be now safe to finally remove this old and useless functionality altogether.