remove root level default_storage_class from capo misc values

default_storage_class was set at root level in {1.6,1.7,current}/misc-units-in-capo.yml helm validates values against the chart schema before templates are resolved, and it fails with error

- default_storage_class: Does not match format 'hostname'

this breaks Sylva 1.6 deployments (Flux 2.6.4 → Helm 3, which enforces format). Sylva 1.7 / main are unaffected only because Flux 2.8.8 → Helm 4 treats format as an annotation under draft 2020-12. (analysed by Claude)

the line is redundant in any case: common/misc-units-in-capo.yml already sets _internal.default_storage_class to the same expression

Closes sylva-projects/sylva-core#4487 (closed)