Removed sphinx roles of some classes and methods,
as the classes are defined in the sample codes, not in Python.

Fixed warnings:
```
howto/descriptor.rst:45: WARNING: py:class reference target not found: Ten [ref.class]
howto/descriptor.rst:218: WARNING: py:class reference target not found: Person [ref.class]
howto/descriptor.rst:218: WARNING: py:class reference target not found: Person [ref.class]
howto/descriptor.rst:256: WARNING: py:class reference target not found: Person [ref.class]
howto/descriptor.rst:340: WARNING: py:class reference target not found: Validator [ref.class]
howto/descriptor.rst:363: WARNING: py:class reference target not found: Validator [ref.class]
howto/descriptor.rst:363: WARNING: py:meth reference target not found: validate [ref.meth]
howto/descriptor.rst:372: WARNING: py:class reference target not found: OneOf [ref.class]
howto/descriptor.rst:374: WARNING: py:class reference target not found: Number [ref.class]
howto/descriptor.rst:378: WARNING: py:class reference target not found: String [ref.class]
howto/descriptor.rst:876: WARNING: py:class reference target not found: Field [ref.class]
howto/descriptor.rst:1143: WARNING: py:func reference target not found: Property [ref.func]
howto/descriptor.rst:1725: WARNING: py:class reference target not found: Object [ref.class]
howto/enum.rst:78: WARNING: py:class reference target not found: Weekday [ref.class]
howto/enum.rst:90: WARNING: py:class reference target not found: Weekday [ref.class]
howto/enum.rst:113: WARNING: py:class reference target not found: Weekday [ref.class]
howto/enum.rst:131: WARNING: py:class reference target not found: Weekday [ref.class]
howto/enum.rst:576: WARNING: py:class reference target not found: Animal [ref.class]
howto/enum.rst:893: WARNING: py:class reference target not found: FloatEnum [ref.class]
```