Remove IncludeCategories from .clang-format (#470)

It was recently noticed that the `IncludeCategories`:
```
IncludeCategories:
  - Regex:           '^<ext/.*\.h>'
    Priority:        2
  - Regex:           '^<.*\.h>'
    Priority:        1
  - Regex:           '^<.*'
    Priority:        2
  - Regex:           '.*'
    Priority:        3
```
In the `.clang-format` are not really necessary as `ext` has no meaning in RAPIDS. This PR removes these.

Note these changes are being made in all repos:
* `cudf`: rapidsai/cudf#9876
* `rmm`: rapidsai/rmm#933
* `cuml`: rapidsai/cuml#4438
* `cugraph`: rapidsai/cugraph#1987

Authors:
  - Conor Hoekstra (https://github.com/codereport)

Approvers:
  - Mark Harris (https://github.com/harrism)

URL: #470

cpp/.clang-format

@@ -72,15 +72,6 @@ ForEachMacros:
   - Q_FOREACH
   - BOOST_FOREACH
 IncludeBlocks: Preserve
-IncludeCategories:
-  - Regex:           '^<ext/.*\.h>'
-    Priority:        2
-  - Regex:           '^<.*\.h>'
-    Priority:        1
-  - Regex:           '^<.*'
-    Priority:        2
-  - Regex:           '.*'
-    Priority:        3
 IncludeIsMainRegex: '([-_](test|unittest))?$'
 IndentCaseLabels: true
 IndentPPDirectives: None

cpp/src/io/shp/polygon_shapefile_reader.cpp

@@ -14,9 +14,10 @@
  * limitations under the License.
  */
 
-#include <ogrsf_frmts.h>
 #include <cuspatial/error.hpp>
 
+#include <ogrsf_frmts.h>
+
 #include <cudf/types.hpp>
 
 #include <memory>

cpp/tests/interpolate/cubic_spline_test.cpp

@@ -28,9 +28,9 @@
 #include <cuspatial/detail/cubic_spline.hpp>
 #include <cuspatial/error.hpp>
 
+#include <string>
 #include <sys/time.h>
 #include <time.h>
-#include <string>
 
 struct CubicSplineTest : public cudf::test::BaseFixture {
 };