fix(ggally_cor): remove unsupported use argument

[vinouselouane]

The ggally_cor function previously included a use argument, documented as
being passed to stats::cor(). However, the function internally calls
stats::cor.test(), which does not accept a use parameter.

As a result, passing use = "complete.obs" or use = "pairwise.complete.obs"
had no effect on the calculation, potentially misleading users who expected
the missing data handling behavior of cor().

This commit:

• Removes the use argument from the function signature and its documentation.
• Updates the function to always handle missing values using the default
  behavior of cor.test(), which excludes incomplete observations pairwise.
• Adds a @details section in the documentation to clarify how missing values
  are treated.

This change brings the implementation in line with the actual behavior and
avoids confusion about NA handling in ggally_cor().