debian: Delete files that were generated by gtk-doc during dist

If we `make dist` with gtk-doc-tools 1.28 from Debian 10 'buster' (the
current stable release), it generates a libcapsule-docs.xml in which
generation of tree_index.sgml is mandatory. This is incompatible with
building this non-GObject library with gtk-doc-tools >= 1.30, in which
tree_index.sgml is only generated if the library contains at least one
GObject type.

Most projects treat the libcapsule-docs.xml generated by gtk-doc as
a template and commit it to git, but in this project we don't
particularly want to maintain it and would prefer to keep regenerating
it during build.

Signed-off-by: Simon McVittie <smcv@collabora.com>
6 jobs for master in 5 minutes and 26 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
manual build:arch #60848
allowed to fail manual
passed build:asan #60844

00:03:13

failed build:bionic #60847

00:05:25

failed build:buster #60846

00:05:12

failed build:stretch #60845

00:05:21

passed build:ubsan #60843

00:03:17

 
Name Stage Failure
failed
build:bionic Test
libcapsule-0.20200908.0.1-d825 archives ready for distribution: 
libcapsule-0.20200908.0.1-d825.tar.gz
================================================================
fatal: Not a valid object name: 'origin/pristine-tar'.
/bin/sh: 1: pristine-tar: not found
Reading package lists...
E: You must put some 'source' URIs in your sources.list
ERROR:deb-build-snapshot:Command 'cd /tmp/build-snapshot.QpQlVMSPMk && apt-get --download-only source libcapsule' returned non-zero exit status 100.
ERROR: Job failed: exit code 1
failed
build:buster Test
libcapsule-0.20200908.0.1-d825 archives ready for distribution: 
libcapsule-0.20200908.0.1-d825.tar.gz
================================================================
fatal: Not a valid object name: 'origin/pristine-tar'.
/bin/sh: 1: pristine-tar: not found
Reading package lists...
E: You must put some 'source' URIs in your sources.list
ERROR:deb-build-snapshot:Command 'cd /tmp/build-snapshot.qeP7Sn7MR8 && apt-get --download-only source libcapsule' returned non-zero exit status 100.
ERROR: Job failed: exit code 1
failed
build:stretch Test
libcapsule-0.20200908.0.1-d825 archives ready for distribution: 
libcapsule-0.20200908.0.1-d825.tar.gz
================================================================
fatal: Not a valid object name: 'origin/pristine-tar'.
/bin/sh: 1: pristine-tar: not found
Reading package lists...
E: You must put some 'source' URIs in your sources.list
ERROR:deb-build-snapshot:Command 'cd /tmp/build-snapshot.2zHV2TofuR && apt-get --download-only source libcapsule' returned non-zero exit status 100
ERROR: Job failed: exit code 1