fix: remove safeguards for parsing colmap data

[Yubel426]

Fix #3611. The safeguards introduced in #3381 makes datasets with non-zero k4 coefficient such as fisheye unusable. When reading camera distortion coefficients such as OPENCV camera, k4 is correctly assigned as the fourth-order radial distortion coefficient rather than the fourth coefficient. Therefore, the safeguards can be removed.

[Yubel426]

@kerrj @jb-ye Would you mind reviewing this PR?

[f-dy]

This is incorrect. #3612 is correct.
colmap's k4, k5 and k6 are still unsupported for the perspective model.
k4 should be supported for the fisheye model.

[Yubel426]

Hi @f-dy, for the perspective models—"SIMPLE_PINHOLE", "PINHOLE", "SIMPLE_RADIAL", and "RADIAL"—their k4 parameter is correctly set to 0 when reading the parameters. I'm not sure what the purpose of this error check is; please let me know if I've misunderstood anything.

It’s possible that k4 doesn’t support fisheye either; a new PR is needed here.

[ishipachev]

@Yubel426
You shouldn't set k4 to zero when you read camera parameters if it has k4 non zero.

Imagine you have some random function over [a;b] and you have a polynom which approximates it: y = a * x^2 + bx + c
You can't "approximate" it further to linear model just by dropping a and having something like y = bx + c.

Instead you need to build a proper new approximation, not just dropping one coefficient from the reacher one.