There is no need for sphinx build dependencies in the arch build.

The motivation for this change was python3-sphobjinv having
a transitive dependency on Rust which isn't available on alpha,
but the change makes sense in general.

The python3-recommonmark build dependency is dropped intentionally due to
https://github.com/open-mpi/ompi/commit/cf52ea933b0f76a8fe1ffcec2c12a218577a6e29


diff -Nru openmpi-6.0.0~rc2/debian/control openmpi-6.0.0~rc2/debian/control
--- openmpi-6.0.0~rc2/debian/control	2026-10-03 17:38:00.000000000 +0300
+++ openmpi-6.0.0~rc2/debian/control	2026-10-03 18:01:56.000000000 +0300
@@ -24,13 +24,14 @@
  libfabric-dev [!hurd-any],
  libnuma-dev [!hurd-any],
  libpsm2-dev [amd64],
- python3-sphinx-rtd-theme:native,
- python3-sphobjinv,
- sphinx,
- python3-recommonmark:native,
  default-jdk [!hurd-any !alpha !ppc64 !sparc64] <!nojava>,
  chrpath,
  flex,
+Build-Depends-Indep:
+ dh-sequence-sphinxdoc,
+ python3-sphinx-rtd-theme:native,
+ python3-sphobjinv,
+ sphinx,
 Build-Conflicts:
  libmpich-dev,
  libopenmpi-dev,
diff -Nru openmpi-6.0.0~rc2/debian/rules openmpi-6.0.0~rc2/debian/rules
--- openmpi-6.0.0~rc2/debian/rules	2026-10-03 17:38:00.000000000 +0300
+++ openmpi-6.0.0~rc2/debian/rules	2026-10-03 18:01:56.000000000 +0300
@@ -79,7 +79,7 @@
 export USER=username

 %:
-	dh $@ --with sphinxdoc
+	dh $@

 extra_flags = \
 		--disable-silent-rules \