NH-3884 - Preliminary clean-up of TypeFactory

 * Removing undue global thread sync and concurrency checks, the concurrent dictionary handles already that.
 * Removing useless singleton pattern, likely a remnant of Java port.

Author: fredericDelaporte

src/NHibernate/SqlTypes/SqlTypeFactory.cs

@@ -79,7 +79,6 @@ public static StringClobSqlType GetStringClob(int length)
 			return GetTypeWithLen<StringClobSqlType>(length, l => new StringClobSqlType(l));
 		}
 
-		[MethodImpl(MethodImplOptions.Synchronized)]
 		public static SqlType GetSqlType(DbType dbType, byte precision, byte scale)
 		{
 			return GetTypeWithPrecision(dbType, precision, scale);