ci: Remove unused dependency installs

`linux-headers-generic` isn't needed, sparc no longer needs the
dependencies to prepare for using qemu-system, and g++ isn't needed for
platforms that have a C-only libc.

Author: tgross35

ci/docker/loongarch64-unknown-linux-gnu/Dockerfile

@@ -6,7 +6,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     gcc-14-loongarch64-linux-gnu \
     libc6-dev \
     libc6-dev-loong64-cross \
-    linux-headers-generic \
     qemu-user
 
 ENV CARGO_TARGET_LOONGARCH64_UNKNOWN_LINUX_GNU_LINKER=loongarch64-linux-gnu-gcc-14 \

ci/docker/riscv64gc-unknown-linux-gnu/Dockerfile

@@ -6,7 +6,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     gcc-riscv64-linux-gnu \
     libc6-dev \
     libc6-dev-riscv64-cross \
-    linux-headers-generic \
     qemu-system-riscv64 \
     qemu-user
 

ci/docker/sparc64-unknown-linux-gnu/Dockerfile

@@ -2,19 +2,12 @@ FROM ubuntu:25.04
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
     ca-certificates \
-    cpio \
-    curl \
     gcc \
     gcc-sparc64-linux-gnu \
-    ipxe-qemu \
     libc6-dev \
     libc6-dev-sparc64-cross \
-    linux-libc-dev-sparc64-cross \
-    openbios-sparc \
-    p7zip-full \
     qemu-system-sparc64 \
-    qemu-user \
-    seabios
+    qemu-user
 
 ENV CARGO_TARGET_SPARC64_UNKNOWN_LINUX_GNU_LINKER=sparc64-linux-gnu-gcc \
     CARGO_TARGET_SPARC64_UNKNOWN_LINUX_GNU_RUNNER="qemu-sparc64 -L /usr/sparc64-linux-gnu" \

ci/docker/x86_64-unknown-linux-gnu/Dockerfile

@@ -2,10 +2,8 @@ FROM ubuntu:25.04
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
     ca-certificates \
-    g++ \
     gcc \
-    libc6-dev \
-    linux-headers-generic
+    libc6-dev
 
 RUN apt search linux-headers
 RUN ls /usr/src