Remove use of enum for constants

This was a useful trick back in the days, but in C++17 it is not needed
any more. The constexpr expression is more clear in its intent.

See for instance here:
https://stackoverflow.com/questions/37259807/static-constexpr-int-vs-old-fashioned-enum-when-and-why

Author: joto

src/export/export_format_pg.cpp

@@ -32,13 +32,8 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 #include <string>
 
-enum {
-    initial_buffer_size = 1024U * 1024U
-};
-
-enum {
-    flush_buffer_size = 800U * 1024U
-};
+static constexpr std::size_t initial_buffer_size = 1024UL * 1024UL;
+static constexpr std::size_t flush_buffer_size = 800UL * 1024UL;
 
 ExportFormatPg::ExportFormatPg(const std::string& /*output_format*/,
                                const std::string& output_filename,