Remove the extra SD_WEBP macro define. Since now we move to a dedicated repo

[dreampiggy]

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• I have searched for a similar pull request in the project and found none

• I have updated this branch with the latest master to avoid conflicts (via merge from master or rebase)

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This merge request fixes / reffers to the following issues: ...

Pull Request Description

In previous version of SDWebImage, we define a macro SD_WEBP to indicate that we supports WebP image and link libwebp third party dependency. Using the macro can avoid actually generate the unused class symbols and files for user who don't need webp (Which may increase the framework binary size).

However, now, we move the SDImageWebPCoder into this dedicated repo. So this means, people who add dependency to our new repo indeed want WebP supports. Those SD_WEBP macro define does not do anything.

It's now time to remove them, to avoid unused extra process for some user (For example, in the previous version, if you need to build a embedded framework link the SDWebImage/WebP, you also to inherit this SD_WEBP macro in the Xcode project)