[Variant] remove VariantMetadata::dictionary_size

[alamb]

Unrelated but these public methods on VariantMetadata are redundant:

arrow-rs/parquet-variant/src/variant/metadata.rs

Lines 210 to 213 in 03a837e

	/// The number of metadata dictionary entries
	pub fn len(&self) -> usize {
	self.dictionary_size()
	}

arrow-rs/parquet-variant/src/variant/metadata.rs

Lines 288 to 291 in 03a837e

	/// Get the dictionary size
	pub const fn dictionary_size(&self) -> usize {
	self.dictionary_size as _
	}

Originally posted by @friendlymatthew in #7943 (comment)

[codephage2020]

take

[alamb]

label_issue.py automatically added labels {'parquet'} from #7943