Remove unused code (dotnet#70624)

* Remove unused AllowZeroAllocator and
DefaultAllocator.

* Remove static_assert_n.

* Remove the native Regex implementation and replace
with a minimal version from TPOP.

Add attribution in third party notices.

THIRD-PARTY-NOTICES.TXT

@@ -1099,3 +1099,13 @@ LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+License notice for code from The Practice of Programming
+-------------------------------
+
+Copyright (C) 1999 Lucent Technologies
+
+Excerpted from 'The Practice of Programming
+by Brian W. Kernighan and Rob Pike
+
+You may use this code for any purpose, as long as you leave the copyright notice and book citation attached.

src/coreclr/inc/iallocator.h

@@ -29,48 +29,4 @@ class IAllocator
     virtual void  Free(void* p) = 0;
 };
 
-// This class wraps an allocator that does not allow zero-length allocations,
-// producing one that does (every zero-length allocation produces a pointer to the same
-// statically-allocated memory, and freeing that pointer is a no-op).
-class AllowZeroAllocator: public IAllocator
-{
-    int m_zeroLenAllocTarg;
-    IAllocator* m_alloc;
-
-public:
-    AllowZeroAllocator(IAllocator* alloc) : m_alloc(alloc) {}
-
-    void* Alloc(size_t sz)
-    {
-        if (sz == 0)
-        {
-            return (void*)(&m_zeroLenAllocTarg);
-        }
-        else
-        {
-            return m_alloc->Alloc(sz);
-        }
-    }
-
-    void* ArrayAlloc(size_t elemSize, size_t numElems)
-    {
-        if (elemSize == 0 || numElems == 0)
-        {
-            return (void*)(&m_zeroLenAllocTarg);
-        }
-        else
-        {
-            return m_alloc->ArrayAlloc(elemSize, numElems);
-        }
-    }
-
-    virtual void Free(void * p)
-    {
-        if (p != (void*)(&m_zeroLenAllocTarg))
-        {
-            m_alloc->Free(p);
-        }
-    }
-};
-
 #endif // _IALLOCATOR_DEFINED_