Remove obsolete db_type defined environment

db_type as a compilation define was first introduced in 2006 in d0e2ac9,
but was removed in 2015 in 25791cf.

Nowadays: ./configure --enable-mssql will set {mssql, true} in vars.config;
That is read by ejabberd.app.src.script to set enabled_backends;
which is consulted by test/suite.erl to test only available backends.

configure.ac

@@ -143,10 +143,10 @@ esac],[if test "x$lua" = "x"; then lua=false; fi])
 AC_ARG_ENABLE(mssql,
 [AS_HELP_STRING([--enable-mssql],[use Microsoft SQL Server database (default: no, requires --enable-odbc)])],
 [case "${enableval}" in
-  yes) db_type=mssql; mssql=true ;;
-  no)  db_type=generic; mssql=false ;;
+  yes) mssql=true ;;
+  no)  mssql=false ;;
   *) AC_MSG_ERROR(bad value ${enableval} for --enable-mssql) ;;
-esac],[db_type=generic; if test "x$mssql" = "x"; then mssql=false; fi])
+esac],[if test "x$mssql" = "x"; then mssql=false; fi])
 
 AC_ARG_ENABLE(mysql,
 [AS_HELP_STRING([--enable-mysql],[enable MySQL support (default: no)])],
@@ -283,7 +283,6 @@ esac
 AC_SUBST(roster_gateway_workaround)
 AC_SUBST(new_sql_schema)
 AC_SUBST(full_xml)
-AC_SUBST(db_type)
 AC_SUBST(odbc)
 AC_SUBST(mssql)
 AC_SUBST(mysql)

rebar.config

@@ -107,7 +107,6 @@
             {if_version_below, "23", {d, 'USE_OLD_PG2'}},
             {if_version_below, "24", {d, 'COMPILER_REPORTS_ONLY_LINES'}},
             {if_version_below, "24", {d, 'SYSTOOLS_APP_DEF_WITHOUT_OPTIONAL'}},
-            {if_var_match, db_type, mssql, {d, 'mssql'}},
             {if_var_false, debug, no_debug_info},
             {if_var_true, debug, debug_info},
             {if_var_true, elixir, {d, 'ELIXIR_ENABLED'}},

vars.config.in

@@ -21,7 +21,6 @@
 %% Macros
 {roster_gateway_workaround, @roster_gateway_workaround@}.
 {full_xml, @full_xml@}.
-{db_type, @db_type@}.
 {debug, @debug@}.
 {new_sql_schema, @new_sql_schema@}.