[Misc] Remove more deprecated APIs

[izackwu]

Related issue = close #3590

This PR further removes more deprecated APIs:

• ti.complex_kernel
• ti.complex_kernel_grad
• ti.outer_product(a, b)
• ti.cross(a, b
• ti.dot(a, b)
• ti.normalized(a)
• use_unified_memory
• ti.determinant(a)
• ti.tr(a)
• a.inversed()
• to_numpy(as_vector)

Also, for other related repos:

• taichi_elements [misc] remove 'use_unified_memory' taichi_elements#85
• difftaichi

After this PR, there should be almost no deprecated APIs in the codebase:

❯ rg "deprecated" python
# This is for testing only, and it raises RuntimeError immediately to alert users, so it's okay to keep it.
python/taichi/main.py
658:            'ti test is deprecated. Please run `python tests/run_tests.py` instead.'

# This is a quite recent change, and according to its original code review comments, we need to keep it
# See https://github.com/taichi-dev/taichi/pull/2774/files#r693818926
python/taichi/__init__.py
22:deprecated_names = {'SOA': 'Layout.SOA', 'AOS': 'Layout.AOS'}
24:    for name, alter in deprecated_names.items():
29:        if attr in deprecated_names:
31:                f'ti.{attr} is deprecated. Please use ti.{deprecated_names[attr]} instead.',
34:            exec(f'{attr} = {deprecated_names[attr]}')

# This is nothing :)
python/taichi/tools/__init__.py
16:    'deprecated',

# This is `deprecated` function definition, maybe we will need it in the future
# to mark other functions as outdated
python/taichi/tools/util.py
81:def deprecated(old, new, warning_type=DeprecationWarning):
82:    """Mark an API as deprecated.
91:        >>> @deprecated('ti.sqr(x)', 'x**2')
102:            msg = f'{old} is deprecated. Please use {new} instead.'

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[strongoier]

LGTM! Thanks!