ARROW-6818: [DOC] Remove reference to Apache Drill design docs

As mentioned in ARROW-6818, it's unclear why there is a link to the Apache Drill design doc for Value Vectors. It seems to me it could make one think that design document applies to the arrow format.

Closes #9442 from martinblostein/format-remove-drill-reference-ARROW-6818

Authored-by: martinblostein <martin.blostein@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>
diff --git a/docs/source/format/Columnar.rst b/docs/source/format/Columnar.rst
index 8b93fe4..a42e863 100644
--- a/docs/source/format/Columnar.rst
+++ b/docs/source/format/Columnar.rst
@@ -1204,10 +1204,6 @@
 data, these custom vectors should be converted to a type that exist in
 the Arrow spec.
 
-References
-----------
-* Apache Drill Documentation - `Value Vectors`_
-
 .. _Flatbuffers: http://github.com/google/flatbuffers
 .. _Flatbuffers protocol definition files: https://github.com/apache/arrow/tree/master/format
 .. _Schema.fbs: https://github.com/apache/arrow/blob/master/format/Schema.fbs
@@ -1217,4 +1213,3 @@
 .. _Endianness: https://en.wikipedia.org/wiki/Endianness
 .. _SIMD: https://software.intel.com/en-us/cpp-compiler-developer-guide-and-reference-introduction-to-the-simd-data-layout-templates
 .. _Parquet: https://parquet.apache.org/documentation/latest/
-.. _Value Vectors: https://drill.apache.org/docs/value-vectors/