Math: revert ability to convert Vector<1, T> to T.

Doesn't solve problem I hoped it would solve (adding pointer and
Vector<1, size_t> still doesn't compile), breaks GCC 4.7 build in some
crazy way and makes certain previously-working operations (like
operator== on Vector<1, int> and int) ambiguous. Not worth pursuing
further, I think.

This reverts commit ca0892f026.
This reverts commit d6d0fd1890.

src/Magnum/Math/Test/VectorTest.cpp

@@ -214,11 +214,7 @@ void VectorTest::constructOneComponent() {
 
     /* Implicit constructor must work */
     constexpr Vector1 vec = 1.0f;
-    CORRADE_COMPARE(vec, Vector1{1.0f});
-
-    /* Conversion back must also work */
-    Float value = vec;
-    CORRADE_COMPARE(value, 1.0f);
+    CORRADE_COMPARE(vec, Vector1(1));
 }
 
 void VectorTest::constructConversion() {

src/Magnum/Math/Vector.h

@@ -211,11 +211,6 @@ template<std::size_t size, class T> class Vector {
             return Implementation::VectorConverter<size, T, U>::to(*this);
         }
 
-        /** @brief Convert single-component vector to scalar value */
-        template<class U, std::size_t s = size, class = typename std::enable_if<s == 1>::type> constexpr /*implicit*/ operator U() const {
-            return *_data;
-        }
-
         /**
          * @brief Raw data
          * @return One-dimensional array of `size` length.