ARROW-254: remove Bit type as it is redundant with Boolean

The only use of Bit is for the nullability (or validity) vector which is best understood as a boolean type.
We should remove it as it is not used.

Author: Julien Le Dem <julien@dremio.com>

Closes #116 from julienledem/arrow_254_remove_bit_type and squashes the following commits:

1cada12 [Julien Le Dem] ARROW-254: remove Bit type

cpp/src/arrow/ipc/metadata-internal.cc

@@ -99,8 +99,6 @@ static Status TypeFromFlatbuffer(flatbuf::Type type, const void* type_data,
       return Status::Invalid("Type metadata cannot be none");
     case flatbuf::Type_Int:
       return IntFromFlatbuffer(static_cast<const flatbuf::Int*>(type_data), out);
-    case flatbuf::Type_Bit:
-      return Status::NotImplemented("Type is not implemented");
     case flatbuf::Type_FloatingPoint:
       return FloatFromFlatuffer(
           static_cast<const flatbuf::FloatingPoint*>(type_data), out);

format/Message.fbs

@@ -20,9 +20,6 @@ table Union {
   mode: UnionMode;
 }
 
-table Bit {
-}
-
 table Int {
   bitWidth: int; // 1 to 64
   is_signed: bool;
@@ -62,7 +59,6 @@ table JSONScalar {
 
 union Type {
   Int,
-  Bit,
   FloatingPoint,
   Binary,
   Utf8,