Python bt2: value is a write-only property

Python bt2: value is a write-only property

This commit turns 'value' into a write-only property. The
value property is needed to distinguish between:

my_var = bt2.IntegerFieldType(32)(123456)
my_var = 123

my_var = bt2.IntegerFieldType(32)(123456)
my_var.value = 123

The value 'getter' is not necessary since the various field types
implement the interfaces needed to act as native Python types
(collections.abc.Sequence, collections.abc.Mapping, numbers.Integral,
etc.)

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>