sagemathgh-40801: Remove magic `# sage_setup` comments and their handling
    
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Since sagelib (and sage-the-distro) are no longer using sage_setup,
these can go.

I was wondering of also the distutil comments in cython files are safe
to be deleted, or are they implicitly used in downstream user packages?

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### ⌛ Dependencies

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URL: sagemath#40801
Reported by: Tobias Diez
Reviewer(s): Dima Pasechnik

Author: Release Manager

src/bin/sage

@@ -482,9 +482,6 @@ usage_advanced() {
         echo "  --fixdoctests file.py"
         echo "                      -- Run doctests and replace output of failing doctests"
         echo "                         with actual output."
-        echo "  --fixdistributions <files|dirs>"
-        echo "                      -- Check or update '# sage_setup: distribution'"
-        echo "                         directives in source files"
     fi
     echo "  --sh [...]         -- run a shell with Sage environment variables"
     echo "                        as they are set in the runtime of Sage"

src/sage/algebras/steenrod/all.py

@@ -1,4 +1,3 @@
-# sage_setup: distribution = sagemath-combinat
 """
 The Steenrod algebra
 """

src/sage/arith/long.pxd

@@ -1,4 +1,3 @@
-# sage_setup: distribution = sagemath-objects
 # sage.doctest: needs sage.misc.cython
 r"""
 Fast conversion of Python objects to C long

src/sage/arith/numerical_approx.pxd

@@ -1,4 +1,3 @@
-# sage_setup: distribution = sagemath-objects
 # sage.doctest: needs sage.rings.real_mpfr
 
 cpdef inline long digits_to_bits(d) except -1:

src/sage/arith/numerical_approx.pyx

@@ -1,4 +1,3 @@
-# sage_setup: distribution = sagemath-objects
 # sage.doctest: needs sage.rings.real_mpfr
 r"""
 Generic numerical approximation function

src/sage/arith/power.pxd

@@ -1,4 +1,3 @@
-# sage_setup: distribution = sagemath-objects
 from cpython.number cimport PyNumber_TrueDivide
 from sage.structure.element cimport Element
 

src/sage/arith/power.pyx

@@ -1,4 +1,3 @@
-# sage_setup: distribution = sagemath-objects
 """
 Generic implementation of powering
 

src/sage/categories/action.pxd

@@ -1,4 +1,3 @@
-# sage_setup: distribution = sagemath-objects
 from sage.structure.element cimport Element
 from sage.categories.morphism cimport Morphism
 from sage.categories.map cimport Map

src/sage/categories/action.pyx

@@ -1,4 +1,3 @@
-# sage_setup: distribution = sagemath-objects
 r"""
 Group, ring, etc. actions on objects
 

src/sage/categories/additive_groups.py

@@ -1,4 +1,3 @@
-# sage_setup: distribution = sagemath-categories
 r"""
 Additive groups
 """