dev-libs/liblinear: Revert "Add IUSE=blas."

dev-libs/liblinear: Revert "Add IUSE=blas."

In commit 22acd3654a599abcfe5457c157b6e2937a0242ec ("Add IUSE=blas.") I
tried to add support for linking against a BLAS implementation as
suggested in liblinear's documentation.

As so far liblinear does not support any helpers for linking against it
as well as a BLAS library in an automated way, enabling USE=blas can
result in linking failures in packages that are unaware of the BLAS
linkage. Until a pkg-config file or other reliable source for library
linkage is provided with liblinear, usage of an external BLAS library
cannot be supported.

Package-Manager: Portage-2.3.71, Repoman-2.3.17
Fixes: https://bugs.gentoo.org/691926
Signed-off-by: Jeroen Roovers <jer@gentoo.org>