Remove implementation of Matrix struct and add exercise placeholder

exercises/practice/matrix/Sources/Matrix/Matrix.swift

@@ -1,27 +1,3 @@
 struct Matrix {
-
-  let rows: [[Int]]
-  let columns: [[Int]]
-
-  init(_ stringRepresentation: String) {
-    var rows = [[Int]]()
-    var columns = [[Int]]()
-
-    let rowItems = stringRepresentation.split(separator: "\n")
-    for item in rowItems {
-      let elements = item.split(separator: " ").compactMap { Int(String($0)) }
-      rows.append(elements)
-    }
-    for i in 0..<rows[0].count {
-      var column = [Int]()
-      for row in rows {
-        column.append(row[i])
-      }
-
-      columns.append(column)
-    }
-
-    self.rows = rows
-    self.columns = columns
-  }
+  // Write your code for the 'Matrix' exercise in this file.
 }