[Variant] Remove superfluous check when validating monotonic offsets

[friendlymatthew]

This is a follow on from #7878

Per the spec, VariantList and VariantMetadata must have offsets encoded in monotonically increasing order.

We currently do this here:

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

Lines 222 to 230 in 387490a

	// Validate offsets are in-bounds and monotonically increasing.
	// Since shallow verification checks whether the first and last offsets are in-bounds,
	// we can also verify all offsets are in-bounds by checking if offsets are monotonically increasing.
	let are_offsets_monotonic = offsets.is_sorted_by(|a, b| a < b);
	if !are_offsets_monotonic {
	return Err(ArrowError::InvalidArgumentError(
	"offsets are not monotonically increasing".to_string(),
	));
	}

and

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

Lines 240 to 248 in 387490a

	// Validate offsets are in-bounds and monotonically increasing.
	// Since shallow validation ensures the first and last offsets are in bounds, we can also verify all offsets
	// are in-bounds by checking if offsets are monotonically increasing.
	let are_offsets_monotonic = offsets.is_sorted_by(|a, b| a < b);
	if !are_offsets_monotonic {
	return Err(ArrowError::InvalidArgumentError(
	"offsets not monotonically increasing".to_string(),
	));
	}

Since we later loop over these offsets to access elements in the respective buffers, any non-monotonic offset range would err when accessing elements.

[alamb]

label_issue.py automatically added labels {'parquet'} from #7906