build: Stop allowing to set deprecated bzip2 compressor as dpkg-deb default

We are trying to get rid of it, so do not make life more difficult by
allowing to set it as the default dpkg-deb compressor.

debian/changelog

@@ -82,6 +82,8 @@ dpkg (1.18.8) UNRELEASED; urgency=medium
     - Preset Last-Update field in patch header template with current time in
       Dpkg::Source::Package::V2. Thanks to Daniel Shahaf <danielsh@apache.org>.
       Closes: #828146
+  * Build system:
+    - Stop allowing to set deprecated bzip2 compressor as dpkg-deb default.
   * Test suite:
     - Bump perlcritic ValuesAndExpressions::RequireNumberSeparators minimum
       to 99999.

m4/dpkg-build.m4

@@ -69,7 +69,7 @@ AC_DEFUN([DPKG_DEB_COMPRESSOR], [
                     [change default dpkg-deb build compressor])],
     [with_dpkg_deb_compressor=$withval], [with_dpkg_deb_compressor=$1])
   AS_CASE([$with_dpkg_deb_compressor],
-    [gzip|xz|bzip2], [:],
+    [gzip|xz], [:],
     [AC_MSG_ERROR([unsupported default compressor $with_dpkg_deb_compressor])])
   AC_DEFINE_UNQUOTED([DPKG_DEB_DEFAULT_COMPRESSOR],
                      [COMPRESSOR_TYPE_]AS_TR_CPP(${with_dpkg_deb_compressor}),